General rule for rounding off

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