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4.15: Mental Math to Divide by Whole Number Powers of Ten

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Do you remember Kailey and the astronomy question about earth? This was presented in an earlier Concept. When Kailey finished with her first astronomy dilemma, she was on to the next one. This one is going to involve dividing by whole number powers of ten. Take a look.

What would the diameter of the earth be if it were 100 times smaller?

The diameter of the earth is 12,756.3 km. Kailey knows that she will need to divide, and she thinks that she can use mental math to do it. Do you think that she is right? Pay close attention during this Concept and you will understand how Kailey can solve this problem.

Guidance

In the last few Concepts, you learned how to use mental math when multiplying, you can use mental math to divide by whole number powers of ten too.

See if you can see the pattern.

2.5 \div 10 & = .25\\2.5 \div 100 & = .025\\2.5 \div 1000 & = .0025

What is the pattern?

When you divide by a power of ten, you move the decimal point to the left according to the number of zeros that are in the power of ten that you are dividing by.

Once you have learned and memorized this rule, you will be able to divide using mental math.

Notice that division is the opposite of multiplication. When we multiplied by a power of ten we moved the decimal point to the right. When we divide by a power of ten, we move the decimal point to the left.

Now it is your turn to practice. Use mental math to divide the following decimals by using powers of ten.

Example A

4.5 \div 10 = _____

Solution: .45

Example B

.678 \div 1000 = _____

Solution: .00678

Example C

87.4 \div 100 = _____

Solution: .874

Now you are ready to help Kailey with her problem about shrinking the earth.

Kailey’s question asks if what the diameter of the earth would be if it were 100 times smaller. To complete this problem, Kailey needs to divide the diameter of the earth by 100. She will move the decimal point two places to the left.

12,756.3 \div 100 = 127.563

Wow! The earth went from being in the ten-thousands to being in the hundreds. Think about how much smaller that is!

Vocabulary

Here are the vocabulary words found in this Concept.

Power of ten
10, 100, 1000, 10,000 - you can think of them as multiples of ten.
Scientific notation
a way to write decimals and numbers by writing a number sentence that shows a power of ten using an exponent.

Guided Practice

Here is one for you to try on your own.

67.89 \div 1000 = _____

Answer

To divide by a power of ten, we will need to move the decimal point in the dividend. Here we are dividing by 1000, so we move the decimal point three places to the left.

Our answer is .06789 .

Video Review

Here is a video for review.

James Sousa Dividing by Powers of Ten

Practice

Directions: Use mental math to divide each decimal by a power of ten.

1. 3.4 \div 10 = ______

2. .67 \div 10 = _____

3. 8.91 \div 10 = _____

4. 12.34 \div 10 = _____

5. 67.89 \div 10 = _____

6. 67.89 \div 100 = ______

7. 32.10 \div 100 = ______

8. .568 \div 100 = _____

9. 45 \div 100 = ______

10. 235 \div 100 = ______

11. 67.9 \div 1000 = _____

12. 4.545 \div 1000 = _____

13. .457 \div 1000 = _____

14. 44.57 \div 1000 = _____

15. 1234.5 \div 1000 = _____

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Date Created:

Oct 29, 2012

Last Modified:

Aug 18, 2014
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