6th Grade Math Textbook, Fundamentals

 &KDSWHU

4-6

Update your skills. See pages 411 IX, 412 XIII.

Multiply Decimals

Objective To multiply positive and negative decimals •To multiply more than two

decimal factors

James, who has started a healthy diet, wants to

move to Mars. That is because on Mars, a person’s

weight is 0.38 as great as it is on Earth. If James

weighs 122.5 pounds on his scale in San Diego,

what would he weigh on Mars?

To find out his weight

on Mars, multiply:

0.38 • 122.5

First estimate by rounding:

0.38 • 122.5

0.4 • 1000.40  40

Then multiply the original numbers

to find the actual product.

James would weigh 46.55 pounds

on Mars.

Check:46.55 is close to the estimate of 40, so the answer is reasonable.

Round each

factor to the

greatest place

that does not

have a zero in it.

Key Concept

Multiplying Decimals

• Multiply as you would with integers.


• Count the total numberof decimal places
  in both factors.


• Place the decimal point in the answer.
  The product has the same number of decimal places
  as the combined number of decimal places in both factors.

Multiply: 0.5 • (0.18)

• First estimate: 0.5 • (0.2) 0.1


• Then multiply the original numbers to find
  the actual product.

So 0.5 • (0.18) 0.090, and 0.090 0.09.

Check:0.09 is close to the estimate of 0.1, so the answer is reasonable.

1

The product of two rational numbers

with unlikesigns is negative.

Write a placeholder zero to fill the decimal place.

122.5

0.38

9800

 3675

46.550

1 decimal place

2 decimal places

3 decimal places

0.18

0.5

0.090

2 decimal places

1 decimal place

3 decimal places

Think

0.18 is close to 0.2,

and half of 0.2 is 0.1.

Remember:When you multiply a number

by 100, you move the decimal point

in the number 2 places to the right.