Basic Mathematics for College Students

Chapter 6 Summary and Review 583

To find 50% of a number,divide the number
by 2.

Estimate the answer: What is 50% of 1,442,957?

We use 1,400,000 as an approximation of 1,442,957 because it is even,

divisible by 2, and ends with many zeros.

50% of Because 50% of 1,400,000 

1,400,000 2 700,000.

1,442,957700,000



To find 25% of a number,divide the number
by 4.

Estimate the answer: What is 25% of 21.004?

We use 20 as an approximation because it is close to 21.004 and

because it is divisible by 4.

25% of 21.004 5 Because 25% of 20 ^204  5.

To find 5% of a number,find 10% of the number
by moving the decimal point in the number one
place to the left. Then, divide that result by 2.

Estimate the answer: What is 5% of 36,150?

First, we find 10% of 36,150:

10% of

We use 3,600 as an approximation of this result because it is close to

3,615 and because it is even, and therefore divisible by 2. Next, we

divide the approximation by 2 to estimate 5% of 36,150.

Thus, 5% of 36,1501,800.

3,600

2

1,800

36,15 03,615

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

TIPPING Estimate the 15% tip on a dinner costing $88.55.

To simplify the calculations, we will estimate the cost of the $88.55

dinner to be $90. Then, to estimate the tip, we find 10% of $90 and 5%

of $90, and add.

10% of $90 is $9

5% of $90 (half as much as 10% of $90)

The tip should be $13.50.

$9

 $4.50

$13.50





To find 200% of a number,multiply the number
by 2. A similar approach can be used to find
300% of a number, 400% of a number, and so on.

Estimate the answer: What is 200% of 3.509?

To estimate 200% of 3.509, we will find 200% of 4. We use 4 as an

approximation because it is close to 3.509 and it makes the

multiplication by 2 easy.

200% of 3.509 8 Because 200% of 4  2  4 8.

Sometimes we must approximate the percent,to
estimate an answer.

QUALITY CONTROL In a production run of 145,350 ceramic tiles,

3% were found to be defective. Estimate the number of defective

tiles.

To estimate 3% of 145,350, we will find 1% of 150,000, and multiply

the result by 3. We use 150,000 as the approximation because it is close

to 145,350 and it ends with several zeros.

3% of Because 1% of 150,000 1,500 and

3 1,500 4,500.

There were about 4,500 defective tiles in the production run.

145,3504,500