PA
Pre-Algebra View details
5. Divide Decimals
Continue to next lesson
Lesson
Exercises
Tests
Chapter 3
5. 

Divide Decimals

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 more expand_more
Student Learning Objectives:
  • Estimate quotients using compatible numbers
  • Divide decimal numbers
9 Theory slides
10 Exercises - Grade E - A
Each lesson is meant to take 1-2 classroom sessions
Divide Decimals
Slide of 9
Dividing decimals is similar to dividing whole numbers. The main difference is where to place the decimal point in the quotient. This lesson explores how to divide decimals correctly.

Catch-Up and Review

Here are a few recommended readings before getting started with this lesson.

Challenge

LaShay's Piggy Bank

LaShay is saving money to buy a used 4K drone.

Help her answer the following questions to improve her saving plan.

a

How many $100 bills does she need to make $3000?

b

How many $10 bills does she need to make $300?

c

How many dimes does she need to make $3?

d

How many pennies does she need to make $0.30?

Discussion

Compatible Numbers

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.

Using compatible numbers 600 and 10 to estimate the given quotient
The compatible numbers we chose are not the only options. Any numbers that are easily divisible and close to the original numbers are worth exploring. For example, rather than rounding 612 to 600, let's try 630. Since 63 is a multiple of 9, 630 is divisible by 9. The estimate for the quotient is then 630÷ 9 = 70.

Using compatible numbers 630 and 9 to estimate the given quotient
Example

Estimating the Number of Coins in a Pile

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.

Hint

Divide the total weight of all the money in the pile by the weight of a single coin. Then, round the weight of a dime to a whole number.

Solution

To find the exact number of coins in the pile, we would divide the total weight of all the coins by the weight of one coin. 137.79 ÷ 2.27 However, we only want to estimate the number of coins in the pile, not find the exact number. We can estimate the number of coins using compatible numbers. First, round the divisor to a whole number. The digit in the tenths place is 2, which is less than 5, so we will round the divisor 2. ccc 137.79 & ÷& 2. 27 & & ↓ & & 2 Recall that compatible numbers are numbers that make doing mental math more straightforward. One way to know that the numbers will be compatible is to round the dividend to a number that is a multiple of 2. In this case, we will round it to 140 because it is close to 137.79 and is a multiple of 2. ccc 137.79 & ÷& 2.27 ↓ & & ↓ 140 & ÷ & 2 The quotient of 140 and 2 is 70. The pile contains about 70 coins.
Discussion

Dividing Decimals

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.

1
Multiply the Divisor and Dividend by the Same Power of 10
expand_more
When dividing decimals, the divisor is changed to a whole number. We multiply the divisor and the dividend by the same power of 10. First, let's write the division using long division notation. The dividend is 58.46 and the divisor is 3.7.

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.

2
Divide As Usual
expand_more
Start by dividing the whole number part of the dividend by the divisor.

3
Place the Decimal Point and Continue the Division
expand_more
Now place the decimal point in the quotient above the decimal point in the dividend. Then, bring down the digit in the tenths place and continue dividing as usual.

The quotient of 58.46 and 3.7 is 15.8.

Example

How Many Hours Should LaShay Work?

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.

Girl-lawn-mowing.jpg

a

Estimate the number of hours LaShay needs to mow lawns to earn enough to buy the drone.

b

Find the exact number of hours. Round the answer to one decimal place.

Hint

a

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.

b

Start by converting the divisor into a whole number. Use long division to divide the decimals.

Solution

a

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.

b

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.

Long division of 15424 by 925

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.

Example

How Far Does the Drone Go?

LaShay bought a drone.

a

The drone can go 115.45 meters in 5 minutes. How far can the drone go in one minute?

b

LaShay notices a beehive on a tree branch while flying the drone in the backyard of the house.

Beehive-on-tree-branch.jpg

Her grandfather, who used to be a beekeeper, tells LaShay the following.

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.

Hint

b

Multiply the divisor and the dividend by a power of 10.

Solution

a

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.

Dividing 115.45 by 5

The result of the division is 23.09. Therefore, the drone can go 23.09 meters in one minute.

b

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.

Dividing 84000 by 14

This means that there are about 6000 bees in the hive.

Pop Quiz

Finding the Quotient of Two Decimal Numbers

Divide the decimal numbers. Make sure that the decimal point is placed correctly. Round the answer to two decimal places, if necessary.

Quotient of randomly generated decimals
Closure

Money in the Piggy Bank

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.

a

How many $100 bills does she need to make $3000?

b

How many $10 dollar bills does she need to make $300?

c

How many dimes does she need to make $3?

d

How many pennies does she need to make $0.3?

Hint

a

Use long division to divide 3000 by 100.

b

Use long division to divide 300 by 10.

c

A dime is worth $0.10. Divide the decimal 3 by 0.1. Rewrite the expression so that the divisor is 1.

d

A penny is worth $0.01. Divide the decimal 0.3 by 0.01.

Solution

a

The number of $100 bills is found by dividing 3000 by 100.

3000 ÷ 100 Let's use long division to find this quotient.

Dividing 3000 by 100

The quotient is 30. LaShay needs 30 $100 bills to make $3000.

b

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.

Dividing 300 by 10

LaShay can make $300 with 30 $10 bills.

c

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.

Dividing 30 by 0.1

The result is indeed 30. Therefore, 30 dimes are need to make $3.

d

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.



Divide Decimals
Exercises
Edit Lesson
>
2
e
7
8
9
×
÷1
=
=
4
5
6
+
<
log
ln
log
1
2
3
()
sin
cos
tan
0
.
π
x
y