Page 32 - ICSE Math 5

Estimating the Sum

We know that estimation gives an approximate value of the sum that is close to the actual sum.
We can round off a number to any place depending upon the degree of accuracy required.

Thus, estimating the sum means rounding off the given addends to the nearest 10; 100; 1,000
and so on, and then adding the numbers obtained.
Example 5: In a village, there are 12,645 men, 11,378 women and 5,798 children. Estimate
the total population of the village by rounding off the numbers to the nearest
100. Check whether the estimated population is close to the actual population or
not.

Solution: Actual number of men in the village = 12,645
Calculation
Actual number of women in the village = 11,378 1 2 2
1 2 6 4 5
Actual number of children in the village = 5,798
1 1 3 7 8
 Actual population of the village
+ 5 7 9 8
= 12,645 + 11,378 + 5,798 2 9 8 2 1
= 29,821

On rounding off the given numbers to the nearest 100, we get the following
numbers.
Estimated number of men in the village = 12,600 Calculation
1
Estimated number of women in the village = 11,400 1 2 6 0 0
Estimated number of children in the village = 5,800 1 1 4 0 0

 Estimated population of the village + 5 8 0 0
2 9 8 0 0
= 12,600 + 11,400 + 5,800

= 29,800
29,821 when rounded off to the nearest 100 gives 29,800.

So, the estimated population of the village is very close to the actual population of
the village.

Word Problems on Addition

Example 6: A publisher sold 1,46,750 books in January, 1,52,430 books in February and 1,60,000
books in March. How many books did the publisher sell in three months?
Solution: Number of books sold in January = 1,46,750
Calculation
Number of books sold in February = 1,52,430 1 1
1 4 6 7 5 0
Number of books sold in March = 1,60,000
1 5 2 4 3 0
Total number of books sold
+ 1 6 0 0 0 0
= 1,46,750 + 1,52,430 + 1,60,000
4 5 9 1 8 0
= 4,59,180
So, the publisher sold 4,59,180 books in three months.

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