Page 26 - ICSE Math 6

When we estimate we should first identify the place to which rounding is needed. For example, 48,199
when rounded off to the nearest ten thousands is 50,000 and 48,199 when rounded off to the nearest
thousands is 48,000. As 48,000 is closer to 48,199 so we round off 48,199 to the nearest thousands.
Example 17: Estimate each of the following using the general rule.
(a) 730 + 998 (b) 28,292 – 21,496
Solution: (a) 730 + 998
Let’s round off to the nearest hundreds.
730 is rounded off to 700. 700
998 is rounded off to 1,000. + 1,000
Therefore, the estimated sum of 730 and 998 = 700 + 1,000 1,700
= 1,700
(b) 28,292 – 21,496
Let’s round off to the nearest thousands.
28,292 is rounded off to 28,000. 28,000
21,496 is rounded off to 21,000. – 21,000
Therefore, the estimated difference of 28,292 and 21,496 7,000
= 28,000 – 21,000 = 7,000

Example 18: Give a rough estimate by rounding off to the nearest hundreds and also a closer estimate
by rounding off to the nearest tens.
(a) 439 + 334 + 4,317 (b) 3,25,215 – 25,370

Solution: (a) 439 + 334 + 4,317
On rounding off to the nearest hundreds:
439 rounds off to 400 400
334 rounds off to 300 300
4,317 rounds off to 4,300 + 4,300
5,000
Therefore, the estimated sum of 439, 334 and 4,317
= 400 + 300 + 4,300 = 5,000

On rounding off to the nearest tens:
439 rounds off to 440
334 rounds off to 330 440
4,317 rounds off to 4,320 330

Therefore, the estimated sum of 439, 334 and 4,317 + 4,320
= 440 + 330 + 4,320 = 5,090 5,090
(b) 3,25,215 – 25,370
On rounding off to the nearest hundreds:
3,25,215 rounds off to 3,25,200

25,370 rounds off to 25,400 3,25,200
Therefore, the estimated difference of 3,25,215 and 25,370 – 25,400
2,99,800
= 3,25,200 – 25,400 = 2,99,800
On rounding off to the nearest tens:
3,25,215 rounds off to 3,25,220

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