5.2 Decimal Operations

Learning Objectives

By the end of this section, you will be able to:

• Multiply decimals
• Divide decimals
• Use decimals in money applications

Let’s take one more look at the lunch order from the start of Decimals, this time noticing how the numbers were added together.

All three items (sandwich, water, tax) were priced in dollars and cents, so we lined up the dollars under the dollars and the cents under the cents, with the decimal points lined up between them. Then we just added each column, as if we were adding whole numbers. By lining up decimals this way, we can add or subtract the corresponding place values just as we did with whole numbers.

How much change would you get if you handed the cashier a \$20 bill for a \$14.65 purchase? We will show the steps to calculate this in the next example.

Multiply Decimals

Multiplying decimals is very much like multiplying whole numbers—we just have to determine where to place the decimal point. The procedure for multiplying decimals will make sense if we first review multiplying fractions.

Do you remember how to multiply fractions? To multiply fractions, you multiply the numerators and then multiply the denominators.

So let’s see what we would get as the product of decimals by converting them to fractions first. We will do two examples side-by-side in Table 5.3. Look for a pattern.

Once we know how to determine the number of digits after the decimal point, we can multiply decimal numbers without converting them to fractions first. The number of decimal places in the product is the sum of the number of decimal places in the factors.

The rules for multiplying positive and negative numbers apply to decimals, too, of course.

Multiply by Powers of 1010

In many fields, especially in the sciences, it is common to multiply decimals by powers of 10.  Let’s see what happens when we multiply 1.9436 by some powers of 10.

Look at the results without the final zeros. Do you notice a pattern?

The number of places that the decimal point moved is the same as the number of zeros in the power of ten. Table 5.4 summarizes the results.

Table5.4

We can use this pattern as a shortcut to multiply by powers of ten instead of multiplying using the vertical format. We can count the zeros in the power of 10 and then move the decimal point that same of places to the right.

So, for example, to multiply 45.86 by 100, move the decimal point 2 places to the right.

Sometimes when we need to move the decimal point, there are not enough decimal places. In that case, we use zeros as placeholders. For example, let’s multiply 2.4 by 100. We need to move the decimal point 2 places to the right. Since there is only one digit to the right of the decimal point, we must write a 00 in the hundredths place.

Divide Decimals

Just as with multiplication, division of decimals is very much like dividing whole numbers. We just have to figure out where the decimal point must be placed.

To understand decimal division, let’s consider the multiplication problem

Remember, a multiplication problem can be rephrased as a division problem. So we can write

We can think of this as “If we divide 8 tenths into four groups, how many are in each group?” Figure 5.5 shows that there are four groups of two-tenths in eight-tenths. So 0.8÷4=0.2.

Figure 5.5

Using long division notation, we would write

Notice that the decimal point in the quotient is directly above the decimal point in the dividend.

To divide a decimal by a whole number, we place the decimal point in the quotient above the decimal point in the dividend and then divide as usual. Sometimes we need to use extra zeros at the end of the dividend to keep dividing until there is no remainder.

In everyday life, we divide whole numbers into decimals—money—to find the price of one item. For example, suppose a case of 24 water bottles cost \$3.99. To find the price per water bottle, we would divide \$3.99 by 24, and round the answer to the nearest cent (hundredth).

Divide a Decimal by Another Decimal

So far, we have divided a decimal by a whole number. What happens when we divide a decimal by another decimal? Let’s look at the same multiplication problem we looked at earlier, but in a different way.

Remember, again, that a multiplication problem can be rephrased as a division problem. This time we ask, “How many times does 0.2 go into 0.8?” Because (0.2)(4)=0.8, we can say that 0.2 goes into 0.8 four times. This means that 0.8 divided by 0.2 is 4.

We would get the same answer, 4, if we divide 8 by 2, both whole numbers. Why is this so? Let’s think about the division problem as a fraction.

We multiplied the numerator and denominator by 10 and ended up just dividing 8 by 2. To divide decimals, we multiply both the numerator and denominator by the same power of 10 to make the denominator a whole number. Because of the Equivalent Fractions Property, we haven’t changed the value of the fraction. The effect is to move the decimal points in the numerator and denominator the same number of places to the right.

We use the rules for dividing positive and negative numbers with decimals, too. When dividing signed decimals, first determine the sign of the quotient and then divide as if the numbers were both positive. Finally, write the quotient with the appropriate sign.

It may help to review the vocabulary for division:

Use Decimals in Money Applications

We often apply decimals in real life, and most of the applications involving money. The Strategy for Applications we used in The Language of Algebra gives us a plan to follow to help find the answer. Take a moment to review that strategy now.

Be careful to follow the order of operations in the next example. Remember to multiply before you add.

Section 5.2 Exercises

Practice Makes Perfect

In the following exercises, add or subtract.

1. 16.92+7.56
2. 18.37+9.36
3. 256.37−85.49
4. 248.25−91.29
5. 21.76−30.99
6. 15.35−20.88
7. 37.5+12.23
8. 38.6+13.67
9. −16.53−24.38
10. −19.47−32.58
11. −38.69+31.47
12. −29.83+19.76
13. −4.2+(−9.3)
14. −8.6+(−8.6)
15. 100−64.2
16. 100−65.83
17. 72.5−100
18. 86.2−100
19. 15+0.73
20. 27+0.87
21. 2.51+40
22. 9.38+60
23. 91.75−(−10.462)
24. 94.69−(−12.678)
25. 55.01−3.7
26. 59.08−4.6
27. 2.51−7.4
28. 3.84−6.1
Multiply Decimals

In the following exercises, multiply.

1. (0.3)(0.4)
2. (0.6)(0.7)
3. (0.24)(0.6)
4. (0.81)(0.3)
5. (5.9)(7.12)
6. (2.3)(9.41)
7. (8.52)(3.14)
8. (5.32)(4.86)
9. (−4.3)(2.71)
10. (−8.5)(1.69)
11. (−5.18)(−65.23)
12. (−9.16)(−68.34)
13. (0.09)(24.78)
14. (0.04)(36.89)
15. (0.06)(21.75)
16. (0.08)(52.45)
17. (9.24)(10)
18. (6.531)(10)
19. (55.2)(1,000)
20. (99.4)(1,000)
Divide Decimals

In the following exercises, divide.

1. 0.15÷5
2. 0.27÷3
3. 4.75÷25
4. 12.04÷43
5. \$8.49÷12
6. \$16.99÷9
7. \$117.25÷48
8. \$109.24÷36
9. 0.6÷0.2
10. 0.8÷0.4
11. 1.44÷(−0.3)
12. 1.25÷(−0.5)
13. −1.75÷(−0.05)
14. −1.15÷(−0.05)
15. 5.2÷2.5
16. 6.5÷3.25
17. 12÷0.08
18. 5÷0.04
19. 11÷0.55
20. 14÷0.35
Mixed Practice

In the following exercises, simplify.

1. 6(12.4−9.2)
2. 3(15.7−8.6)
3. 24(0.5)+(0.3)2
4. 35(0.2)+(0.9)2
5. 1.15(26.83+1.61)
6. 1.18(46.22+3.71)
7. \$45+0.08(\$45)
8. \$63+0.18(\$63)
9. 18÷(0.75+0.15)
10. 27÷(0.55+0.35)
11. (1.43+0.27)÷(0.9−0.05)
12. (1.5−0.06)÷(0.12+0.24)
13. [\$75.42+0.18(\$75.42)]÷5
14. [\$56.31+0.22(\$56.31)]÷4
Use Decimals in Money Applications

In the following exercises, use the strategy for applications to solve.

1. Spending money Brenda got \$40 from the ATM. She spent \$15.11 on a pair of earrings. How much money did she have left?
2. Spending money Marissa found \$20 in her pocket. She spent \$4.82 on a smoothie. How much of the \$20 did she have left?
3. Shopping Adam bought a t-shirt for \$18.49 and a book for \$8.92 The sales tax was \$1.65. How much did Adam spend?
4. Restaurant Roberto’s restaurant bill was \$20.45 for the entrée and \$3.15 for the drink. He left a \$4.40 tip. How much did Roberto spend?
5. Coupon Emily bought a box of cereal that cost \$4.29. She had a coupon for \$0.55 off, and the store doubled the coupon. How much did she pay for the box of cereal?
6. Coupon Diana bought a can of coffee that cost \$7.99. She had a coupon for \$0.75 off, and the store doubled the coupon. How much did she pay for the can of coffee?
7. Diet Leo took part in a diet program. He weighed 190 pounds at the start of the program. During the first week, he lost 4.3 pounds. During the second week, he had lost 2.8 pounds. The third week, he gained 0.7 pounds. The fourth week, he lost 1.9 pounds. What did Leo weigh at the end of the fourth week?
8. Snowpack On April 1, the snowpack at the ski resort was 4 meters deep, but the next few days were very warm. By April 5, the snow depth was 1.6 meters less. On April 8, it snowed and added 2.1 meters of snow. What was the total depth of the snow?
9. Coffee Noriko bought 4 coffees for herself and her co-workers. Each coffee was \$3.75. How much did she pay for all the coffees?
10. Subway Fare Arianna spends \$4.50 per day on subway fare. Last week she rode the subway 6 days. How much did she spend for the subway fares?
11. Income Mayra earns \$9.25 per hour. Last week she worked 32 hours. How much did she earn?
12. Income Peter earns \$8.75 per hour. Last week he worked 19 hours. How much did he earn?
13. Hourly Wage Alan got his first paycheck from his new job. He worked 30 hours and earned \$382.50. How much does he earn per hour?
14. Hourly Wage Maria got her first paycheck from her new job. She worked 25 hours and earned \$362.50. How much does she earn per hour?
15. Restaurant Jeannette and her friends love to order mud pie at their favorite restaurant. They always share just one piece of pie among themselves. With tax and tip, the total cost is \$6.00. How much does each girl pay if the total number sharing the mud pie is
ⓐ 2?

ⓑ 3?

ⓒ 4?

ⓓ 5?

ⓔ 6?

1. Pizza Alex and his friends go out for pizza and video games once a week. They share the cost of a \$15.60 pizza equally. How much does each person pay if the total number sharing the pizza is
ⓐ 2?

ⓑ 3?

ⓒ 4?

ⓓ 5?

ⓔ 6?

1. Fast Food At their favorite fast food restaurant, the Carlson family orders 4 burgers that cost \$3.29 each and 2 orders of fries at \$2.74 each. What is the total cost of the order?
2. Home Goods Chelsea needs towels to take with her to college. She buys 2 bath towels that cost \$9.99 each and 6 washcloths that cost \$2.99 each. What is the total cost for the bath towels and washcloths?
3. Zoo The Lewis and Chousmith families are planning to go to the zoo together. Adult tickets cost \$29.95 and children’s tickets cost \$19.95. What will the total cost be for 4 adults and 7 children?
4. Ice Skating Jasmine wants to have her birthday party at the local ice skating rink. It will cost \$8.25 per child and \$12.95 per adult. What will the total cost be for 12 children and 3 adults?
Everyday Math

1. Paycheck Annie has two jobs. She gets paid \$14.04 per hour for tutoring at City College and \$8.75 per hour at a coffee shop. Last week she tutored for 8 hours and worked at the coffee shop for 15 hours.
ⓐ How much did she earn?

ⓑ If she had worked all 23 hours as a tutor instead of working both jobs, how much more would she have earned?

1. Paycheck Jake has two jobs. He gets paid \$7.95 per hour at the college cafeteria and \$20.25 at the art gallery. Last week he worked 12 hours at the cafeteria and 5 hours at the art gallery.
ⓐ How much did he earn?

ⓑ If he had worked all 17 hours at the art gallery instead of working both jobs, how much more would he have earned?

Writing Exercises

1. At the 2010 winter Olympics, two skiers took the silver and bronze medals in the Men’s Super-G ski event. Miller’s time was 1 minute 30.62 seconds and Weibrecht’s time was 1 minute 30.65 seconds. Find the difference in their times and then write the name of that decimal.
2. Find the quotient of 0.12÷0.04 and explain in words all the steps taken.

Self Check

ⓐ After completing the exercises, use this checklist to evaluate your mastery of the objectives of this section.

ⓑ After reviewing this checklist, what will you do to become confident for all objectives?