Basic Mathematics for College Students

We can use the shortcuts for finding 10% and 5% of a number to find 15% of a
number.

6.4 Estimation with Percent 555

EXAMPLE (^6) Electricity Usage The average
U.S. household uses 10,656 kilowatt-hours of electricity
each year. Several energy conservation groups would
like each household to take steps to reduce its electricity
usage by 5%. Estimate 5% of 10,656 kilowatt-hours.
(Source:U.S. Department of Energy)
StrategyWe will find 10% of 10,656. Then, we will
divide an approximation of that result by 2.
WHY5% of a number is one-half of 10% of a number.
SolutionFirst, we find 10% of 10,656.
Move the understood decimal
point in 10,656 one place to the left.
We will use 1,066 as an approximation of this result because it is close to 1,065.6 and
because it is even, and, therefore, divisible by 2. Next, we divide the approximation
by 2 to estimate 5% of 10,656.
Divide the approximation of 10% of 10,656 by 2.
Thus, 5% of. A 5% reduction in electricity usage by the average U.S.
household is about 533 kilowatt-hours.

10,656 533

1,066

2

 533

10% of 10,6561,065.6

Self Check 6

Estimate the answer: What is

5% of 24,198?

Now TryProblems 31

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Finding 15% of a Number

To find 15% of a number, find the sum of 10% of the number and 5% of the

number.

EXAMPLE (^7) Tipping As a general rule,
if the service in a restaurant is acceptable, a tip of
15% of the total bill should be left for the server.
Estimate the 15% tip on a $77.55 dinner bill.
StrategyWe will find 10% and 5% of an
approximation of $77.55. Then we will add those
results.
WHYTo find 15% of a number, find the sum of 10% of the number and 5% of the
number.
SolutionTo simplify the calculations, we will estimate the cost of the $77.55
dinner to be $80. Then, to estimate the tip, we find 10% of $80 and 5% of $80, and
add.
10% of $80 is $8
5% of $80 (half as much as 10% of $80)
Add to get the estimated tip.
The tip should be $12.

$8

 $4

$12

Self Check 7

TIPPINGEstimate the 15% tip on

a $29.55 breakfast bill.

Now TryProblems 35 and 75

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