10. Permutations and Combinations in Probability
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Chapter 6
10.
Permutations and Combinations in Probability
Understanding permutations and combinations is pivotal for calculating probabilities, especially in compound events. The lesson delves into the distinction between permutations, where the order of arrangement matters, and combinations, where the order is inconsequential. For instance, when considering the arrangement of letters in a word or the order of runners in a race, permutations come into play. On the other hand, combinations are used when selecting a subset, like choosing books from a list. The lesson also touches upon real-world scenarios, such as determining the likelihood of students sharing the same birthday. Grasping these mathematical concepts can significantly aid in solving complex problems in various fields.
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Lesson Settings & Tools
| | 10 Theory slides |
| | 10 Exercises - Grade E - A |
| | Each lesson is meant to take 1-2 classroom sessions |
Permutations and Combinations in Probability
Slide of 10
In this lesson, the concepts of permutation and combination will be introduced and connected to the computation of probabilities of compound events.
Catch-Up and Review
Here are a few recommended readings before getting started with this lesson.
Challenge
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Creating Arrangements
Vincenzo is playing with the following letters.
Discussion
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Defining Permutations and the Permutation Formula
Many situations involve the rearrangement of a specific set of objects. These are called permutation problems. Below, the definition of permutation and its corresponding formula are discussed.
Concept
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Permutation
A permutation is an arrangement of objects in which the order is important. For example, consider constructing a number using only the digits 4, 5, and 6 without repetitions. Any of the three digits can be picked for the first position, leaving two choices for the second position, then only one choice for the third position.
In this case, there are six possible permutations. 456 465 546 564 645 654 Although all these numbers are formed with the same three digits, the order in which the digits appear affects the number produced. Each different order of the digits creates a different number. The number of permutations can be calculated by using the Fundamental Counting Principle.
Number of permutations&=3*2*1 &⇕ Number of permutations&=6Rule
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Permutation Formula
The number of permutations of n different objects arranged r at a time — denoted as _nP_r — is given by the following formula.
_nP_r=n!/(n-r)!, r≤ n
The exclamation sign in the formula indicates that the factorial of a value must be calculated. As a direct consequence, since 0!=1, when n=r the number of permutations is given by the factorial of n.
_nP_n=n!
An alternative notation for _nP_r is P(n,r).
Proof
Permutation FormulaThe formula can be proven by using the Fundamental Counting Principle. In an arrangement with r elements, there are n choices for the first element, n-1 choices for the second element, n-2 choices for the third element, and so on.
| Position | Number of Choices |
|---|---|
| 1 | n |
| 2 | n-1 |
| 3 | n-2 |
| ... | ... |
| r | (n-r+1) |
_nP_r(n-r)!=n(n-1)...(n-r+1)(n-r)!
Write as a product
_nP_r (n-r)!=n(n-1)...(n-r+1)(n-r)(n-r-1)... (2)(1)
Write as a factorial
_nP_r (n-r)!=n!
DivEqn
.LHS /(n-r)!.=.RHS /(n-r)!.
_nP_r =n!/(n-r)!
Example
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Investigating Permutations in Real-Life Situations
The following cities are the ten most visited cities in Europe.
| Rank | City |
|---|---|
| 1 | London, UK |
| 2 | Paris, France |
| 3 | Istanbul, Turkey |
| 4 | Antalya, Turkey |
| 5 | Rome, Italy |
| 6 | Prague, Czech Republic |
| 7 | Amsterdam, Netherlands |
| 8 | Barcelona, Spain |
| 9 | Vienna, Austria |
| 10 | Milan, Italy |
a Dominika and her friend Heichi are planning to go to Europe next summer. How many different ways can they arrange their trip to see all ten cities?
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b Suppose that they can only visit 3 of the ten places. In how many ways can they do it?
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Hint
a The number of permutations of n out of n is given by the factorial of n.
b Consider the permutation formula for r objects out of n.
Solution
a Because the order in which the cities will be visited is important, the problem can be solved by using permutations. The number of permutations when taking n items out of n is given by the factorial of n.
_nP_n=n!
In this case, since there are ten cities to be visited, the factorial of 10 needs to be calculated.
Therefore, Dominika and Heichi have 3 628 800 different ways to visit all ten most visited cities of Europe.
_nP_n=n!
Substitute
n= 10
_(10)P_(10)= 10!
Write as a product
_(10)P_(10)=10*9*8*8*7*6*5*4*3*2*1
Multiply
Multiply
_(10)P_(10)=3 628 800
b Now suppose that Heichi and Dominika will visit only three places. Since the order of the cities they visit is important, permutations can be used again. The number of permutations when taking r items out of n is given by the following formula. _nP_r=n!/(n-r)!
Of the 10 places, only 3 can be visited. Therefore, the number of permutations of 3 out of 10 needs to be calculated.
There are 720 ways of visit 3 of the ten cities.
_nP_r=n!/(n-r)!
SubstituteII
n= 10, r= 3
_(10)P_3=10!/( 10- 3)!
▼
Evaluate right-hand side
SubTerm
Subtract term
_(10)P_3=10!/7!
Write as a product
_(10)P_3=10*9*8*7!/7!
CancelCommonFac
Cancel out common factors
_(10)P_3=10*9*8*7!/7!
SimpQuot
Simplify quotient
_(10)P_3=10*9*8/1
DivByOne
a/1=a
_(10)P_3=10*9*8
Multiply
Multiply
_(10)P_3=720
Example
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Finding Probabilities Using Permutations
In the 2020 Olympic Games, the competitors of the men's 100 meter freestyle swimming finals came from the following countries.
| Men’s 100 Meter Freestyle Swimming Finals | |
|---|---|
| United States | Australia |
| Russia | France |
| South Korea | Italy |
| Hungary | Romania |
a If there were no ties, in how many different ways could the gold, silver, and bronze medals have been awarded?
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b If all athletes have the same athletic ability, what is the probability that the Italian swimmer wins the gold medal, the French swimmer the silver medal, and the Australian swimmer the bronze medal? Approximate the answer to three decimals.
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Solution
a Because the order in which the medals are awarded is essential, the problem can be solved by using permutations. The number of permutations when taking r items out of n is given by the following formula.
_nP_r=n!/(n-r)!
SubstituteII
n= 8, r= 3
_8P_3=8!/( 8- 3)!
▼
Evaluate right-hand side
SubTerm
Subtract term
_8P_3=8!/5!
Write as a product
_8P_3=8*7*6*5!/5!
CancelCommonFac
Cancel out common factors
_8P_3=8*7*6*5!/5!
SimpQuot
Simplify quotient
_8P_3=8*7*6/1
DivByOne
a/1=a
_8P_3=8*7*6
Multiply
Multiply
_8P_3=336
b Recall that the probability of an event is the ratio of the number of favorable outcomes to the number of possible outcomes.
The number of favorable outcomes is the number of ways the Italian, French, and Australian athletes can win the gold, silver, and bronze medals, respectively. Although there is only one way for the order of the first three positions, there are several ways for the order of the remaining five positions. All these are favorable outcomes.
| Example Favorable Outcomes | ||
|---|---|---|
| Italy | Italy | Italy |
| France | France | France |
| Australia | Australia | Australia |
| United States | Hungary | South Korea |
| Russia | Romania | Russia |
| South Korea | Russia | United States |
| Hungary | United States | Hungary |
| Romania | South Korea | Romania |
P(A)=Number of favorable outcomes/Number of possible outcomes
SubstituteValues
Substitute values
P(A)=120/40 320
ReduceFrac
a/b=.a /120./.b /120.
P(A)=1/336
FracToDiv
a/b=a÷ b
P(A)=0.002976...
RoundDec
Round to 3 decimal place(s)
P(A)≈ 0.003
Discussion
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Defining Combinations and the Combination Formula
In other situations, only the selected objects are important, not the order in which they come. These problems are called combination problems. Below, the definition of combination and its corresponding formula are developed.
Concept
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Combination
A combination is a selection of objects in which the order is not important. Combinations focus on the selected objects. For example, consider choosing two different ingredients for a salad from five unique options in a salad bar.
Rule
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Combination Formula
The number of combinations of n different objects taken r at a time — denoted as _nC_r — is given by the following formula.
_nC_r=n!/r!(n-r)!, r≤ n
The exclamation mark in the formula indicates that the factorial of the value should be calculated. As a direct consequence of the above formula, since 0!=1, when n=r the number of combinations is 1.
_nC_n=1
An alternative notation for _cC_r is C(n,r).
Proof
The formula can be proven by using the Permutation Formulas. _nP_r=n!/(n-r)! and _rP_r=r! Let _nC_r be the number of combinations of n objects chosen r at a time. By the Fundamental Counting Principle, the product of _nC_r by _rP_r equals the number of permutations of r objects out of n. _nC_r* _rP_r = _nP_r ⇓ _nC_r* r!= n!/(n-r)! Finally, by applying the Division Property of Equality, the Combination Formula is obtained.
_nC_r* r!= n!/(n-r)! ⇕ _nC_r=n!/r!(n-r)!
Example
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Investigating Combinations in Real-Life Situations
Kriz is going on vacation next month and wants to pack 4 books from their must-read list. Each of the books belongs to one of the following genres.
| Kriz’s List of Books By Genres | |
|---|---|
| Fantasy | Romance |
| Mystery | Fiction |
| Biography | Graphic Novel |
| Drama | History |
| Western | Poetry |
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Hint
The order in which the books are selected is not crucial.
Solution
As long as 4 books are selected, the order is not important. Therefore, the different ways in which Kriz can select 4 books can be found by using combinations. The number of combinations when selecting r items out of n is given by the following formula.
_nC_r=n!/r!(n-r)!
By substituting 10 for n and 4 for r, the number of combinations can be calculated.
There are 210 ways in which Kriz can select 4 books to pack from their must-read list.
_nC_r=n!/r!(n-r)!
SubstituteII
n= 10, r= 4
_(10)C_4=10!/4!( 10- 4)!
▼
Evaluate right-hand side
SubTerm
Subtract term
_(10)C_4=10!/4!*6!
Write as a product
_(10)C_4=10*9*8*7*6!/4!*6!
CancelCommonFac
Cancel out common factors
_(10)C_4=10*9*8*7*6!/4! *6!
SimpQuot
Simplify quotient
_(10)C_4=10*9*8*7/4!
Write as a product
_(10)C_4=10*9*8*7/4*3*2*1
Multiply
Multiply
_(10)C_4=5040/24
CalcQuot
Calculate quotient
_(10)C_4=210
Example
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Finding Probabilities Using Combinations
Kriz has decided that they will select 5 of their books at random instead of 4. However, they would prefer to bring at least one book from the fantasy, mystery, and drama genres. What is the probability of them choosing these three genres if the selection pool consists of 10 books from 10 different genres? Write the answer in percentage form rounded to 1 decimal place.
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Hint
The probability of an event is the ratio of the number of favorable outcomes to the number of possible outcomes.
Solution
The probability of an event is the ratio of the number of favorable outcomes to the number of possible outcomes.
_nC_r=n!/r!(n-r)!
SubstituteII
n= 10, r= 5
_(10)C_5=10!/5!( 10- 5)!
▼
Evaluate right-hand side
SubTerm
Subtract term
_(10)C_5=10!/5!*5!
Write as a product
_(10)C_5=10*9*8*7*6*5!/5!*5!
CancelCommonFac
Cancel out common factors
_(10)C_5=10*9*8*7*6*5!/5!*5!
SimpQuot
Simplify quotient
_(10)C_5=10*9*8*7*6/5!
Write as a product
_(10)C_5=10*9*8*7*6/5*4*3*2*1
Multiply
Multiply
_(10)C_5=30240/120
CalcQuot
Calculate quotient
_(10)C_5=252
_nC_r=n!/r!(n-r)!
SubstituteII
n= 7, r= 2
_7C_2=7!/2!( 7- 2)!
▼
Evaluate right-hand side
SubTerm
Subtract term
_7C_2=7!/2!*5!
Write as a product
_7C_2=7*6*5!/2!*5!
CancelCommonFac
Cancel out common factors
_7C_2=7*6*5!/2!*5!
SimpQuot
Simplify quotient
_7C_2=7*6/2!
2!=2
_7C_2=7*6/2
Multiply
Multiply
_7C_2=42/2
CalcQuot
Calculate quotient
_7C_2=21
3 of the 5 books selected are a fantasy, a mystery, and a drama.By substituting the number of favorable outcomes and the number of possible outcomes into the Probability Formula, P(A) can be found.
P(A)=Number of favorable outcomes/Number of possible outcomes
SubstituteII
Number of favorable outcomes= 21, Number of possible outcomes= 252
P(A)=21/252
ReduceFrac
a/b=.a /21./.b /21.
P(A)=1/12
FracToDiv
a/b=a÷ b
P(A)=0.083
RoundDec
Round to 3 decimal place(s)
P(A)≈ 0.083
WritePercent
Convert to percent
P(A)≈ 8.3 %
Example
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The Birthday Problem
Magdalena teaches algebra to a group of 10 students. While making a list to track their attendance, she wonders whether at least 2 students have the same birthday. For simplicity, suppose that all years have exactly 365 days.
a What is the probability that at least two students have the same birthday? Approximate the answer to two decimal places.
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b If there were 30 students, what would be the probability that at least 2 students the same birthday? Approximate the answer to two decimal places.
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Hint
a Consider the Complement Rule of Probability.
b Substitute 30 for n and 2 for r in the expression found in Part A.
Solution
a Note that the phrase
Now, the numerator will be simplified.
The above expression represents the permutations of 365 taking n at a time. _(365)P_n=365!/(365-n)!
Therefore, P(A') is the quotient of _(365)P_n and 365^n.
P(A')=_(365)P_n/365^n
The Complement Rule of Probability states that P(A) is the difference between 1 and P(A'). By using the expression found for P(A'), an expression for P(A) can be found.
P(A)=1-P(A') ⇕ P(A)=1-_(365)P_n/365^n
To apply the formula, P(A') will be calculated first.
Finally, by subtracting P(A') from 1, the probability of A can be calculated.
at leastmeans that any outcome with 2 or more students with the same birthday is a favorable outcome. Therefore, the following outcomes are all the possible favorable outcomes.
| Favorable Outcomes | ||
|---|---|---|
| 2 have the same birthday | 3 have the same birthday | 4 have the same birthday |
| 5 have the same birthday | 6 have the same birthday | 7 have the same birthday |
| 8 have the same birthday | 9 have the same birthday | 10 have the same birthday |
Suppose that instead of 10, there are n students. Let A be the event that at least 2 students out of n have their birthday on the same day. If n is greater than 365, then certainly at least 2 students would share their birthday. For the sake of the example, n is assumed to be less than or equal to 365. A=&At least 2students out of nhave &their birthday on the same day Finding the probability of every possible outcome — all nine outcomes in the table above — means finding nine different probabilities. The opposite of at least 2 students having their birthday on the same day is that there are no students with their birthday on the same day. This is the complement of A written as A'. A'=&No one of the nstudents &shares a birthday The Complement Rule of Probability can help to deal with this situation. To do so, a general expression will be found for no students from a group of n having the same birthday. Then, P(A) can be calculated.
General Expression For P(A') and P(A)
Suppose that the group consists of only 2 students. The first student can have their birthday on any day. The probability of the second student not having their birthday on the same day is the ratio of 364 to 365. Therefore, this expression is the probability of the two students not having their birthday on the same day. Probability of2 Students Not Having Their Birthday on the Same Day 364/365 Suppose that a third student is added to the group. The probability that this student has their birthday on a different day from the previous two is the ratio of 363 to 365. By the Multiplication Rule of Probability, the probability that no students out of these three have their birthdays on the same day can be found. Probability of3 Students Not Having Their Birthday on the Same Day 364/365*363/365 ⇔ 364*363/365^2 By following the same reasoning, the probability that n students do not share a birthday, P(A'), can be written. P(A')= 364/365*363/365*...365-n+1/365_(n-1 times) [2.5em] ⇕ [1em] P(A')= 364*363*...*(365-n+1)/365^(n-1) The obtained expression will now be simplified.P(A')=364*363*...*(365-n+1)/365^(n-1)
▼
Simplify right-hand side
IdPropMult
Identity Property of Multiplication
P(A')=1* 364*363*...*(365-n+1)/365^(n-1)
OneToFrac
Rewrite 1 as 365/365
P(A')=365/365* 364*363*...*(365-n+1)/365^(n-1)
MultFrac
Multiply fractions
P(A')=365* 364*363*...*(365-n+1)/365* 365^(n-1)
MultBasePow
a*a^m=a^(1+m)
P(A')=365* 364*363*...*(365-n+1)/365^n
365* 364*363*...*(365-n+1)
▼
Simplify
IdPropMult
Identity Property of Multiplication
365* 364*363*...*(365-n+1) * 1
OneToFrac
Rewrite 1 as (365-n)!/(365-n)!
365* 364*363*...*(365-n+1) * (365-n)!/(365-n)!
MoveLeftFacToNum
a*b/c= a* b/c
365* 364*363*...*(365-n+1) * (365-n)!/(365-n)!
Substitute
365* 364*363*...*(365-n+1) * (365-n)!= 365!
365!/(365-n)!
Calculating P(A) if n=10
In this case, A is the event that at least 2 students out of 10 have their birthday on the same day.
| A | A' |
|---|---|
| At leat 2 students out of 10 have their birthday on the same day. | No one of the 10 students shares a birthday. |
P(A')=_(365)P_n/365^n
Substitute
n= 10
P(A')=_(365)P_(10)/365^(10)
▼
Evaluate right-hand side
Substitute
_(365)P_(10)= 365!/(365-10)!
P(A')=365!(365-10)!/365^(10)
SubTerm
Subtract term
P(A')=365!355!/365^(10)
Write as a product
P(A')=365*364*...*356*355!355!/365^(10)
CancelCommonFac
Cancel out common factors
P(A')=365*364*...*356*355!355!/365^(10)
SimpQuot
Simplify quotient
P(A')=365*364*...*3561/365^(10)
DivByOne
a/1=a
P(A')=365*364*...*356/365^(10)
UseCalc
Use a calculator
P(A')=0.883051...
RoundDec
Round to 2 decimal place(s)
P(A')≈ 0.88
b It was found that if A is the event that at least 2 out of n students have their birthday on the same day, P(A) is given by the following equation.
P(A)=1-_(365)P_n/365^n
By substituting 30 for n, P(A) can be found for a group of 30 students.
The probability that at least 2 students out of 30 have their birthday on the same day is about 0.71. What is more, and extremely curious, only 57 people are required to bring the probability that at least two people have the same birthday up to 0.99.
P(A)=1-_(365)P_n/365^n
Substitute
n= 30
P(A)=1-_(365)P_(30)/365^(30)
▼
Evaluate right-hand side
Substitute
_(365)P_(30)= 365!/(365-30)!
P(A)=1-365!(365-30)!/365^(30)
SubTerm
Subtract term
P(A)=1-365!335!/365^(30)
Write as a product
P(A)=1-365*364...*336*335!335!/365^(30)
CancelCommonFac
Cancel out common factors
P(A)=1-365*364...*336*335!335!/365^(30)
SimpQuot
Simplify quotient
P(A)=1-365*364...*3361/365^(30)
DivByOne
a/1=a
P(A)=1-365*364...*336/365^(30)
UseCalc
Use a calculator
P(A)=1-0.293683...
SubTerm
Subtract term
P(A)=0.706317...
RoundDec
Round to 2 decimal place(s)
P(A)≈ 0.71
Closure
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Creating Arrangements
Permutations and combinations can be used in many situations. Understanding these mathematical concepts can help solve many intricate problems. With this in mind, reconsider the problem in which Vincenzo wants to create arrangements by using the following letters.
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Hint
Begin by calculating the number of ways of selecting 3 vowels and 4 consonants. The order of the arrangements is essential.
Solution
Because the arrangements consist of 3 vowels and 4 consonants, they have 7 letters. An example arrangement is shown.
_nC_r=n!/r!(n-r)!
SubstituteII
n= 5, r= 3
_5C_3=5!/3!( 5- 3)!
▼
Evaluate right-hand side
SubTerm
Subtract term
_5C_3=5!/3!*2!
Write as a product
_5C_3=5*4*3!/3!*2!
CancelCommonFac
Cancel out common factors
_5C_3=5*4*3!/3!*2!
SimpQuot
Simplify quotient
_5C_3=5*4/2!
Multiply
Multiply
_5C_3=20/2!
2!=2
_5C_3=20/2
CalcQuot
Calculate quotient
_5C_3=10
_nC_r=n!/r!(n-r)!
SubstituteII
n= 6, r= 4
_6C_4=6!/4!( 6- 4)!
▼
Evaluate right-hand side
SubTerm
Subtract term
_6C_4=6!/4!*2!
Write as a product
_6C_4=6*5*4!/4!*2!
CancelCommonFac
Cancel out common factors
_6C_4=6*5*4!/4!*2!
SimpQuot
Simplify quotient
_6C_4=6*5/2!
Multiply
Multiply
_6C_4=30/2!
2!=2
_6C_4=30/2
CalcQuot
Calculate quotient
_6C_4=15
Permutations and Combinations in Probability
Exercises
Maya is playing a tile-based word game and has three tiles with the following letters.
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Because we want the number of different arrangements of the given letters, we have a permutation problem. The permutation of n items out of n is given by the factorial of n. In this case, since we have 3 tiles, we need to calculate the factorial of 3 to find the different arrangements. 3!=3* 2* 1 ⇔ 3!= 6 Therefore, there are 6 different ways of arranging the given letters. Additionally, since the number of letters is still small enough, we can visualize the possible arrangements using a tree diagram.
We can use a graphing calculator to determine how many arrangements there are. Start by entering a 3 on your graphing calculator.
Next, push MATH, scroll to the fourth menu, PRB. Finally, scroll to the fourth row and push ENTER twice for the answer.
The probability of an event A is given by the ratio of the favorable outcomes to the number of possible outcomes.
P(A)=Number of favorable outcomes/Number of possible outcomes
The number of possible outcomes is given by all the different arrangements of the given letters. We have found that there are 6 possible arrangements. Therefore, the number of possible outcomes is 6.
Number of possible outcomes= 6
Note that with the three letters, only one word is possible.
Then, the number of favorable outcomes is 1. Therefore, the probability of randomly making a word using all the three letters is given by the ratio of 1 to 6. P(Form a word)=1/6
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Six students are running for president of the student council. Before the voting begins, they are each allowed to give a speech to the rest of the school. In how many different orders can the students be asked to give their speeches?
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We need to determine the number of possible orders for the speech. Because of this, we have a permutation problem. Recall that the number of permutations of n items out of n is given by the factorial of n. Since there are 6 candidates, we need to calculate the factorial of 6. 6!=6* 5* 4* 3* 2* 1 ⇔ 6!= 720 There are 720 different combinations to arrange the student's speeches. This cannot be easily visualized by hand with a tree diagram.
The problem can also be solved by using a graphing calculator. Begin by entering 6 on the calculator.
Next, push MATH, scroll to the fourth menu, PRB. Finally, scroll to the fourth row and push ENTER twice.
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Paulina and a group of friends are going to the high school football game this weekend. To cheer them on, each of them has printed out a letter in the team's name, MOHAWKS,
on large cards that they will hold up during the game.
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Since we want the number of the different arrangements of letters, we have a permutation problem. Note that there are seven unique letters in the word MOHAWKS.
From the permutation formula we know that the number of permutations of n items out of n is given by the factorial of n.
_nP_n=n!
Therefore, we need to calculate the factorial of 7.
7!=7* 6* 5* 4* 3* 2* 1 ⇕ 7!= 5040
There are 5040 ways of arranging the letters.
The probability of an event A is given by the ratio of the number of favorable outcomes to the number of possible outcomes. P(A)=Number of favorable outcomes/Number of possible outcomes The number of possible outcomes is given by the different possible arrangements of the seven letters. Previously, we found that there are 5040 different arrangements. Number of possible outcomes= 5040 Additionally, because all the letters are unique, there is only one possible arrangement that form the team's name. Therefore, we have only one favorable outcome. Number of favorable outcomes= 1 The probability that the cards still spell out the correct name given that they were mixed is given by the ratio of 1 to 5040. P(correct name)=1/5040
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Ten runners are about to race. How many ways could the runners finish in the top three?
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In a race, the order in which the runners finish the race is important. This means that we have a permutation problem. Because we are looking for the top three, we need to calculate the permutations when taking 3 out of 10. To do so, we can use the permutation formula. _nP_r=n!/(n-r)! Therefore, we need to substitute 3 for r and 10 for n into the formula.
There are 720 ways to arrange the top three spots.
We can use the calculator to determine the number of permutations. Start by entering the number of people in the race on your graphing calculator, which is 10.
Next, push MATH, scroll to the fourth menu, PRB. Then, scroll to the second row and push ENTER.
Finally, by entering the number of people to be selected, we can determine the number of permutations when n=10 and r=3.
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In how many ways can the letters in the following word be arranged?
RANGE
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In the word RANGE,
there are 5 unique letters. Because we want the different arrangements that can be formed with this letter set, we are looking for permutations. The number of permutations of n objects out of n is given by the factorial of n.
_nP_n=n!
Therefore, we need to calculate the factorial of 5.
There are 120 possible arrangements.
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Permutations and Combinations in Probability
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