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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