General rule says
- For a number in tens, we round off to tens.
- For a number in hundred, we round off to hundred.
- For a number in thousands, we round off to thousands.
Eg :
479 × 81
According to general rule,
- 479 is rounded off to nearest hundred
∴ 479 → 500
- 81 is rounded off to nearest ten
∴ 81 → 80
So, Estimated Product = 500 × 80
= 5 × 100 × 8 × 10
= (5 × 8) × 1000
= 40 × 1000
= 40000
Let's look at some more examples
Eg 2:
37 × 1889
According to general rule,
- 37 is rounded off to nearest ten
∴ 37 → 40
- 1889 is rounded off to nearest thousand
∴ 1889 → 2000
So, Estimated Product = 40 × 2000
= 4 × 10 × 2 × 1000
= (4 × 2) × 10000
= 8 × 10000
= 80000
Eg 3:
232 + 4821
According to general rule,
-
232 is rounded off to nearest hundred
∴ 232 → 200
-
4821 is rounded off to nearest thousand
∴ 4821 → 5000
So, Estimated Sum = 200 + 5000
= 5200
Eg 4:
1089 + 6871
According to general rule,
-
1089 is rounded off to nearest thousand,
∴ 1089 → 1000
-
6871 is rounded off to nearest thousand
∴ 6871 → 7000
∴ Estimated Sum = 1000 + 7000
= 8000
Eg 5:
9560 − 421
According to general rule,
-
9560 is rounded off to nearest thousand,
∴ 9560 → 10,000
-
421 is rounded off to nearest hundred,
∴ 421 → 400
∴ Estimated Difference = 10,000 – 400
= 9600
