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

  1. Chapter 1 Class 6 Knowing our Numbers
  2. Concept wise

Transcript

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 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

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Davneet Singh
Davneet Singh is a graduate from Indian Institute of Technology, Kanpur. He has been teaching from the past 8 years. He provides courses for Maths and Science at Teachoo. You can check his NCERT Solutions from Class 6 to 12.