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This lesson focuses on teaching the methods for dividing decimals, a crucial skill in mathematics. It introduces the concept of compatible numbers, which are numbers that make mental calculations easier. For instance, if you're trying to divide 612 by 9, rounding 612 to 600 makes the math simpler. The lesson also delves into the traditional method of long division for more complex calculations. These techniques are particularly useful in everyday scenarios where quick mental math is needed, such as calculating discounts or splitting bills. Whether you're a student looking to improve your math skills or an adult wanting to make everyday calculations easier, this lesson offers valuable insights.
Show less Show more expand_more| Student Learning Objectives: |
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| | 9 Theory slides |
| | 10 Exercises - Grade E - A |
| | Each lesson is meant to take 1-2 classroom sessions |
LaShay is saving money to buy a used 4K drone.
Help her answer the following questions to improve her saving plan.
How many $100 bills does she need to make $3000?
How many $10 bills does she need to make $300?
How many dimes does she need to make $3?
How many pennies does she need to make $0.30?
Compatible numbers are numbers that help make doing mental math more manageable. Compatible numbers are used to get quick and rough approximations, not precise answers. Consider finding the estimation of the following quotient. 612 ÷ 9 It might seem difficult to divide these numbers using mental math. However, both numbers can be rounded to compatible numbers — 612 to 600 and 9 to 10. An estimate for 612 ÷ 9 can then be found mentally by dividing 600 by 10, which is equal to 60. That means the given quotient is about 60.
A pile of identical coins weighs 137.79 grams.
Each coin weighs 2.27 grams. Use compatible numbers to estimate the number of coins in the pile.
The process of dividing decimals is similar to the process of dividing whole numbers using long division. The main difference between the processes is how the decimal point is placed when dividing decimals. Consider dividing the following decimal numbers. 58.46 ÷ 3.7 Follow these steps to divide the decimals.
In this case, the divisor must be multiplied by 10^1 so that the decimal point moves to the right and the divisor becomes a whole number. We also multiply the dividend by the same power of 10.
The quotient of 58.46 and 3.7 is 15.8.
LaShay needs $279.99 to buy a drone. She has already saved $125.75. She decided to mow lawns to make the rest of money she needs. She charges $9.25 per hour.
Estimate the number of hours LaShay needs to mow lawns to earn enough to buy the drone.
Find the exact number of hours. Round the answer to one decimal place.
Subtract the amount that LaShay saved from the price of the drone. Use compatible numbers to estimate the quotient of the difference and 9.25.
Start by converting the divisor into a whole number. Use long division to divide the decimals.
Let's start by finding the amount of money LaShay still needs. We will find the difference between the price of the drone, $279.99, and the amount LaShay has saved, $125.75.
cr & 2 7 9 . 9 9 - & 1 2 5 . 7 5 & 1 5 4 . 2 4 We can find the number of hours LaShay needs to work to earn this difference by dividing it by the hourly wage, 9.25. 154.24 ÷ 9.25 Let's use compatible numbers to estimate the quotient. First, round the divisor to a whole number. For 9.25, the digit in the tenths place is 2. This digit is less than 5, so we round 9.25 to 9. ccc 154.24 & ÷ & 9.25 & & ↓ & & 9 To find a number that is compatible with 9, we need a number that is a multiple of 9 and close to 154.24. We can use either 90 or 180. Since 180 is closer to our number, let's replace 154.24 with 180. ccc 154.24 & ÷ & 9.25 & ↓ & & ↓ & 180 & ÷ & 9 & = 20 The quotient of 180 and 9 is 20. Therefore, LaShay needs to work about 20 hours to earn $154.24. Note that the goal here is to find an answer quickly, which lead us to an imprecise answer. The exact value will probably be different from this estimation.
The quotient from Part A that represents the number of hours LaShay needs to work.
154.24 ÷ 9.25 [0.6em] ⇓ [0.6em] 9.25 ) 154.24 This is a division of two decimal numbers. The first step when dividing decimals is to convert the divisor into a whole number. Since the divisor has two decimal places, we multiply it and the dividend by 10^2 to keep the expression balanced.
Now we have a division of two whole numbers. Use long division to calculate the quotient.
The quotient is 16.674 with a remainder of 550. This means that LaShay has to mow lawns for about 16.7 hours to save enough money to buy the drone.
LaShay bought a drone.
The drone can go 115.45 meters in 5 minutes. How far can the drone go in one minute?
LaShay notices a beehive on a tree branch while flying the drone in the backyard of the house.
Her grandfather, who used to be a beekeeper, tells LaShay the following.
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So you want to approximate the number of bees in that hive, huh? Well, count the number of bees that leave the hive in one minute. Then, multiply it by 3 and divide by 0.014. |
LaShay counts 28 bees that leave the hive in one minute. Find the number of bees in the hive.
Is the divisor a whole number? Use long division to divide the decimals.
Multiply the divisor and the dividend by a power of 10.
To find how far the drone can travel, let's divide 115.45 by 5.
115.45 ÷ 5 [0.3em] ⇓ [0.3em] 5 ) 115.45 The steps for dividing a decimal by a whole number are the same as the steps for dividing whole numbers. Remember to place the decimal point in the quotient above the decimal point in the dividend.
The result of the division is 23.09. Therefore, the drone can go 23.09 meters in one minute.
We want find the number of bees in the hive. The number of bees that leave the hive in one minute is multiplied by 3, then divided by 0.014.
(Number of Bees * 3 ) ÷ 0.014 LaShay counted 28 bees leaving the hive in one minute. Let's multiply this number by 3. cr & 2 0.265cm & 2 8 * & 3 & 8 4 Now we divide this number by 0.014 using long division. 84 ÷ 0.014 [0.3em] ⇓ [0.3em] 0.014 ) 84 In this case, the divisor is a decimal number. The first step to dividing decimals is to convert the divisor into a whole number. Since the divisor has three decimal places, we will multiply it by 10^3. Make sure the divided is multiplied by the same number as the divisor to keep the expression equivalent.
Now that this is a division of two whole numbers, we can divide as usual.
This means that there are about 6000 bees in the hive.
Divide the decimal numbers. Make sure that the decimal point is placed correctly. Round the answer to two decimal places, if necessary.
When dividing decimals, the divisor is converted into a whole number by multiplying it by a power of 10. However, the dividend must also be multiplied by the same power of 10 to keep the value of the quotient the same. Let's reconsider LaShay's piggy bank.
How many $100 bills does she need to make $3000?
How many $10 dollar bills does she need to make $300?
How many dimes does she need to make $3?
How many pennies does she need to make $0.3?
Use long division to divide 3000 by 100.
Use long division to divide 300 by 10.
A dime is worth $0.10. Divide the decimal 3 by 0.1. Rewrite the expression so that the divisor is 1.
A penny is worth $0.01. Divide the decimal 0.3 by 0.01.
The number of $100 bills is found by dividing 3000 by 100.
3000 ÷ 100 Let's use long division to find this quotient.
The quotient is 30. LaShay needs 30 $100 bills to make $3000.
Use the same reasoning as in Part A. Divide 300 by 10 to find the number of $10 bills needed to make $300.
300 ÷ 10 This is also equal to 30.
LaShay can make $300 with 30 $10 bills.
A dime is worth $0.10. To find the number of dimes needed to make $3, divide 3 by 0.1.
3 ÷ 0.1 In this case, the divisor is a decimal number. The quotient must be rewritten so that the divisor is a whole number. If we multiply the divisor by 10, it will be a whole number. Remember that we also have to multiply the dividend by the same number. Quotient 3 ÷ 0.1 ⇓ Multiplied by10 30 ÷ 1 When a number is divided by 1, the result will be the number itself. We can confirm this by using long division.
The result is indeed 30. Therefore, 30 dimes are need to make $3.
A penny is worth $0.01 and LaShay wants to make $0.30 with pennies. Let's divide 0.3 by 0.01 to find the number of pennies she needs.
0.3 ÷ 0.01 Like in the previous part, the divisor is a decimal number. Multiply the divisor and the dividend by 100 to make the divisor a whole number. Quotient 0.3 ÷ 0.01 ⇓ Multiplied by100 30 ÷ 1 After multiplication, the quotient becomes the same quotient as in the previous part. Since 30 divided by 1 is 30, the quotient, 0.3 ÷ 0.01, is also 30. 0.3 ÷ 0.01 = 30 This means that LaShay can make $0.3 using 30 pennies. Now that we have completed all of the divisions, let's create a table using the quotients.
| Dividend | Divisor | Quotient |
|---|---|---|
| 3000 | 100 | 30 |
| 300 | 10 | 30 |
| 30 | 1 | 30 |
| 3 | 0.1 | 30 |
| 0.3 | 0.01 | 30 |
The numbers in the first two columns decrease by a factor of 10, but the quotient always stays the same. When the dividend and divisor both increase by the same factor of 10, the quotient remains the same.
Let's take a look at the given quotient. 37 ÷ 7.3 We want to estimate the quotient of a whole number and a decimal number. We will do two things to estimate it.
We use compatible numbers because they are numbers that are more manageable when dividing using mental math. Let's round the divisor. We look at the digit in the tenths place to round it to the nearest whole number.
Now we will look for a number that is around 37 and compatible with 7. The best way to do so is to round 37 to a number that is a multiple of 7. Let's list some multiples of 7. Some Multiples of7 21,28, 35,42,49 The closest multiple of 7 to 37 is 35. Let's round 37 to 35 and then divide 35 by 7.
The given quotient is about 5 by our estimation.
We want to estimate the quotient of two decimal numbers.
11.76 ) 52.9
In this case, the divisor is 11.76 and the dividend is 52.9. We will first round 11.76 to the nearest whole number. Then, we will round 52.9 so that the numbers are compatible. Let's round the divisor. We can round the digit in the tenths place to the nearest whole number.
Now we will look for a number that is around 52.9 and compatible with 12. A good way to do this is by rounding 52.9 to a number that is a multiple of 12. Let's list some multiples of 12. Some Multiples of12 24,36, 48,60,72 The closest multiple of 12 to 52.9 is 48. Let's round 52.9 to 48 and then divide 48 by 12.
The given quotient is about 4 by our estimation.
We want to calculate the given quotient. 8.94 ÷ 15 Here, we need to divide a decimal number by a whole number. We will use long division to calculate it.
Since 8 ones divided by 15 is 0, we write a 0 in the ones place of the quotient. Then we place the decimal point in the quotient directly above its place in the dividend.
We can now continue the division process as usual. We will not stop until the remainder becomes 0.
Therefore, the quotient is 0.596.
We will use long division to calculate the given quotient.
The first digit of the quotient will be in the ones place because 64 contains two groups of 26. After that we can place the decimal point in the quotient directly above its place in the dividend.
We can now continue the division process as usual.
Therefore, the quotient is 2.464.
We are asked to find the value of the following quotient. 7 ÷ 0.14 We will use long division to find it. Notice that the divisor is a decimal number. We will first make it a whole number by multiplying it by a power of 10. Since 0.14 has two decimal places, we multiply it by 10 to the power of 2. We also have to multiply the dividend by the number. Given 7 ÷ 0.14 ⇓ Use Long Division 0.14 ) 7 ⇓ Multiply by 10^2 14 ) 700 Now both numbers are whole numbers. We can perform the division as usual.
The quotient is 50.
We will use long division to calculate the given quotient.
34 ÷ 4.25
Again, we need to multiply the dividend and the divisor by 10^2. This is because the divisor has two decimal places and multiplying by 10^2 moves the decimal point two places to the right.
Given
34 ÷ 4.25
⇓
Use Long Division
4.25 ) 34
⇓
Multiply by 10^2
425 ) 3400
There are eight groups of 425 in 3400. Therefore, the quotient is 8.
We see that the divisor is a decimal number with one decimal place. 1.476 ÷ 0.9 We will make the divisor a whole number by multiplying it by 10. We also have to multiply the dividend by the number. Given 1.476 ÷ 0.9 ⇓ Use Long Division 0.9 ) 1.476 ⇓ Multiply by 10 9 ) 14.76 We can divide 14.76 by 9 as we would with whole numbers. We will place the first digit of the quotient in the ones places because 14 contains one group of 9. After that we place the decimal point in the quotient directly above its place in the dividend.
The given quotient is 1.64.
This time we are given a quotient whose divisor is a decimal with three decimal places.
0.0364 ÷ 0.013
We multiply the dividend and the divisor by 10^3. This is because the divisor has three decimal places and multiplying by 10^3 moves the decimal point three places to the right.
Given
0.0364 ÷ 0.013
⇓
Use Long Division
0.013 ) 0.0364
⇓
Multiply by 10^3
13 ) 36.4
Since 2 * 13 is 26 and 3* 13 is greater than 36, the number in the ones place will be 2. Let's start with writing it!
Let's recall the order of operations. We use the acronym PEMDAS to remember the correct order!
Expressions inside parentheses are evaluated first, followed by exponents, then multiplication and division, and finally addition and subtraction are evaluated last. For the given expression, this means multiplying 6.2 and 10.32 first and then dividing it by 6.45. 6.2 * 10.32 ÷ 6.45 When we multiply two decimals, we ignore any decimal points and multiply as we would with whole numbers. Then we count the number of decimal places in each factor. cr & 1 0 3 2 * & 6 2 & 2 0 6 4 + & 6 1 9 2 0 & 6 3 9 8 4 Since 6.2 has one decimal place and 10.32 has two decimal places, their product will have three decimal places. Then the product of the numbers is 63.984. 6.2 * 10.32 ÷ 6.45 ⇓ 63.984 ÷ 6.45 Our goal is to find this quotient now. But first, we need to convert the divisor into a whole number. We can do it by multiplying the divisor by 10^2. We must multiply the dividend by the same power of 10 so that the value of the quotient remains the same. Given 63.984 ÷ 6.45 ⇓ Use Long Division 6.45 ) 63.984 ⇓ Multiply by 10 645 ) 6398.4 Let's find it!
The expression is equal to 9.92.
We need to follow the order of operations. For the given expression, this means evaluating the expression inside the parentheses completely before dividing.
(2^2-2.34) ÷ 1.6
Even within the expression inside the parentheses, we must follow the order of operations. We will first evaluate the exponent. It is equal to 4.
(4-2.34) ÷ 1.6
To subtract 2.34 from 4, we rewrite 4 as 4.00 and subtract by regrouping.
cr
& 3 9 10
& 4 . 0 0
- & 2 . 3 4
& 1 . 6 6
Now that we found the value of the expression inside the parentheses, we can divide it by 1.6.
(2^2-2.34) ÷ 1.6
⇓
1.66 ÷ 1.6
The divisor is a decimal number. We need to make it a whole number. We multiply the dividend and the divisor by 10 because the divisor has one decimal place.
Given
1.66 ÷ 1.6
⇓
Use Long Division
1.6 ) 1.66
⇓
Multiply by 10
16 ) 16.6
The first digit of the quotient will be 1 since 16 times 1 is 16.
The value of the expression is 1.0375.