1. Knowing Our Numbers Worksheet With Answers Page 17
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25general rule that we follow is, therefore, round off each factor to its greatest place and then multiply
the rounded off factors.
Example 26: Estimate the following products using general rule:
(a) 9,250 × 29
(b) 5,281 × 3,849
(c) 1,291 × 592
(NCERT)
Solution:
(a) 9,250 × 29
Rounding off each factor to its greatest place:
9,250 rounded off to the nearest thousands is 9,000.
29 rounded off to the nearest tens is 30.
9,000
Therefore, estimated product of 9,250 and 29
×
30
= 9,000 × 30
2,70,000
= 2,70,000
(b) 5,281 × 3,849
Rounding off each factor to the nearest thousands:
5,281 is rounded off to 5,000.
3,849 is rounded off to 4,000.
5,000
Therefore, estimated product of 5,281 and 3,849
×
4,000
= 5,000 × 4,000
2,00,00,000
= 2,00,00,000
(c) 1,291 × 592
Rounding off each factor to its greatest place:
1,291 rounded off to the nearest thousands is 1,000.
1,000
592 rounded off to the nearest hundreds is 600.
×
600
Therefore, estimated product of 1,291 and 592
6,00,000
= 1,000 × 600
= 6,00,000
Example 27: Estimate and compare with the actual sum:
(a) 870 + 986
(b) 10,866 + 2,368
(c) 2,345 + 5,679
Solution:
(a) 870 + 986
900
Let’s round off to the nearest hundreds.
+ 1,000
870 is rounded off to 900.
1,900
986 is rounded off to 1,000.
Therefore, estimated sum = 900 + 1,000
870
= 1,900
+ 986
However, actual sum = 870 + 986
1,856
= 1,856
On rounding off 1,856 to its nearest hundreds, it becomes 1,900.
Therefore, estimation is reasonable.
(b) 10,866 + 2,368
Let’s round off to the nearest thousands.
10,866 is rounded off to 11,000.
17
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