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EStimAting tHE Sum

We can estimate the sum of two numbers by rounding off.
Example 1. Find the actual as well as estimated sum of the following numbers:

a. 25 + 34 b. 48 + 73

Solution. a. Actual Sum Estimated Sum b. Actual Sum Estimated Sum
round off round off
2 5 3 0 4 8 5 0
+ 3 4 round off + 3 0 + 7 3 round off + 7 0
5 9 6 0 1 2 1 1 2 0

Example 2. A balloon-seller has 248 red balloons and 173 green balloons. Estimate

the total number of balloons by rounding off to the nearest 10. Is the total number of
balloons more than 400 or less than 400?
Solution. Actual Sum Estimated Sum
round off
2 4 8 2 5 0
+ 1 7 3 round off + 1 7 0

4 2 1 4 2 0
Thus, the balloon-seller has more than 400 balloons.

Exercise 2E

Find the actual sum first and then the estimated sum by rounding off to the nearest 10.
Compare the sums. (one has been done for you.)

round off round off
1. 3 4 3 0 2. 5 1
+ 6 5 round off + 7 0 + 4 3 round off +

9 9 1 0 0
Actual sum < Estimated sum Actual sum ___ Estimated sum
round off round off
3. 4 6 4. 5 6

+ 5 5 round off + + 4 1 round off +

Actual sum ___ Estimated sum Actual sum ___ Estimated sum
round off round off
5. 7 2 6. 5 9
+ 1 1 round off + + 3 5 round off +

Actual sum ___ Estimated sum Actual sum ___ Estimated sum

48 Mathematics 3