Decimal Rounding to Estimate
Sums and Differences
Jen Kershaw, M.ed
Kimberly Hopkins
Jen Kershaw
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Printed: January 11, 2016
AUTHORS
Jen Kershaw, M.ed
Kimberly Hopkins
Jen Kershaw
www.ck12.org
C HAPTER
Chapter 1. Decimal Rounding to Estimate Sums and Differences
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Decimal Rounding to
Estimate Sums and Differences
In this concept, you will learn how to estimate sums and differences of decimals.
Let’s Think About It
Juan goes to the store and purchases two items. One costs $21.95 and the other costs $14.25. He wants to estimate
his total before he gets to the register, but he isn’t sure how to estimate the values. What is an estimate of Juan’s
total?
In this concept, you will learn how to estimate sums and differences of decimals.
Guidance
To estimate means to find an answer that is close to but not exact. It is a reasonable answer to a problem.
A sum is the answer of an addition problem. A difference is the answer of a subtraction problem.
The easiest way to estimate a sum or a difference of decimals is to round the decimal. If you round the decimal to
the nearest whole number, you can complete the problem using mental math or at least simplify the problem so that
finding an answer is easier.
Let’s look at an example.
Estimate 15.7 + 4.9 = _____
In this problem, you only want to estimate the sum. You can do that by rounding each decimal to the nearest whole
number.
In 15.7, the place being rounded is the 5. Look at the 7 and round up.
15.7 becomes 16
In 4.9, the place being rounded is the 4. Look at the 9 and round up.
4.9 becomes 5
Next, rewrite the problem.
16 + 5 = 21
The answer is that 15.7 + 4.9 is approximately 21.
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You can also use rounding when estimating sums of larger numbers.
Let’s look at another example.
Estimate 350.12 + 120.78 = _____
Round each number to the nearest whole number to find a reasonable estimate.
350.12 becomes 350
120.78 becomes 121
350 + 121 = 471
The answer is that 350.12 + 120.78 is approximately 471.
Now let’s look at estimations with subtraction.
You can work on these problems in the same way, by rounding.
Estimate 45.78 - 22.10 = _____
45.78 rounds to 46
22.10 rounds to 22
46 - 22 = 24
The answer is that 45.78 - 22.10 is approximately 24.
Guided Practice
Estimate $588.80 - $310.11 = _____
First, round each decimal to the nearest whole dollar.
$588.80 becomes $589
$310.11 becomes $310
Next, subtract the rounded values.
$589 - $310 = $279
The answer is that $588.80 - $310.11 is approximately $279.
Examples
Example 1
Estimate 2.67 + 3.88 = _____
First, round each decimal to the nearest whole number.
2.67 becomes 3
3.88 becomes 4
Next, add the rounded values.
3+4=7
Then, write the approximate sum.
7
The answer is that 2.67 + 3.88 is approximately 7.
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Chapter 1. Decimal Rounding to Estimate Sums and Differences
Example 2
Estimate 56.7 - 22.3 = _____
First, round each decimal to the nearest whole number.
56.7 becomes 57
22.3 becomes 22
Next, subtract the rounded values.
57 - 22 = 35
Then, write the approximate difference.
35
The answer is that 56.7 - 22.3 is approximately 35.
Example 3
Estimate $486.89 - $25.22 = _____
First, round each decimal to the nearest whole dollar.
$486.89 becomes $487
$25.22 becomes $25
Next, subtract the rounded values.
$487 - $25 = $462
Then, write the approximate difference.
$462
The answer is that $486.89 - $25.22 is approximately $462.
Follow Up
Remember Juan who wants to estimate the total cost of his purchases? Estimate the sum of $21.95 and $14.25.
First, round each decimal to the nearest whole dollar.
$21.95 becomes $22
$14.25 becomes $14
Next, add the rounded values.
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$22 + $14 = $36
Then, write the approximate sum.
$36
The answer is that $21.95 + $14.25 is approximately $36.
Video Review
MEDIA
Click image to the left or use the URL below.
URL: http://www.ck12.org/flx/render/embeddedobject/54781
Explore More
Estimate each sum or difference by rounding.
1. 56.32 + 23.12 = _____
2. 18.76 + 11.23 = _____
3. 14.56 + 76.98 = _____
4. 11.12 + 54.62 = _____
5. 33.24 + 45.32 = _____
6. 18.97 + 15.01 = _____
7. 22.43 + 11.09 = _____
8. 4.52 + 3.21 = _____
9. 19.19 + 27.75 = _____
10. 87.12 + 88.90 = _____
11. 67.19 - 33.12 = _____
12. 88.92 - 33.10 = _____
13. 76.56 - 3.45 = _____
14. 65.72 - 11.12 = _____
15. 77.34 - 43.02 = _____
16. 88.02 - 11.10 = _____
17. 89.32 - 18.03 = _____
18. 24.67 - 10.10 = _____
19. 37.82 - 14.20 = _____
20. 55.88 - 44.22 = _____
21. 334.56 - 125.86 = _____
22. 456.11 + 112.18 = _____
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Chapter 1. Decimal Rounding to Estimate Sums and Differences
Answers for Explore More Problems
To view the Explore More answers, open this PDF file and look for section 3.15.
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