Page 328 - Basic College Mathematics with Early Integers

S E C T I O N 4.3 I MULTIPLYING DECIMALS AND CIRCUMFERENCE OF A CIRCLE 305

Example 13 Circumference of a Circle PRACTICE 13
Find the circumference of a
Find the circumference of a circle whose radius is 5 inches. Then use the approxi- circle whose radius is 11 meters.
mation 3.14 for to approximate the circumference.p
Then use the approximation
Solution: Let r = 5 in the formula C = 2pr. 3.14 for to approximate thisp
circumference.
C = 2pr
#
= 2p 5 5 inches
= 10p

Next, replace with the approximation 3.14.
p
C = 10p
(is approximately) ¡ L 10(3.14)
= 31.4
The exact circumference or distance around the circle is 10p inches, which is
approximately 31.4 inches.
Work Practice 13

Objective Solving Problems by Multiplying Decimals

The solutions to many real-life problems are found by multiplying decimals. We
continue using our four problem-solving steps to solve such problems.

Example 14 Finding the Total Cost of Materials for a Job PRACTICE 14
A biology major is fertilizing
A college student is hired to paint a billboard with paint costing $2.49 per quart.
her personal garden. She uses
If the job requires 3 quarts of paint, what is the total cost of the paint?
5.6 ounces of fertilizer per
Solution: square yard.The garden
measures 60.5 square yards.
1. UNDERSTAND. Read and reread the problem.The phrase “total cost” might
How much fertilizer does
make us think addition, but since this problem requires repeated addition, let’s
she need?
multiply.
2. TRANSLATE.
cost per number
In words: total cost is quart of times of
paint quarts
T T T T T
Translate: total cost = 2.49 * 3
3. SOLVE.We can estimate to check our calculations.The number 2.49 rounds to
2 and 2 * 3 = 6.

2.49
* 3
7.47
4. INTERPRET. Check your work. Since 7.47 is close to our estimate of 6, our
answer is reasonable. State your conclusion:The total cost of the paint is $7.47.
Work Practice 14
Answers
13. 22p m L 69.08 m
14. 338.8 oz