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Estimating differences

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The question

371 − 103

371−103
370−103
370−100
270
  1. Do not work it out exactly. Round each number first. 371 is nearer 370 than 380, so to the nearest ten it becomes 370.
  2. 103 is nearer 100 than 110, so to the nearest ten it becomes 100.
  3. Now it is easy enough to do in your head: 370 − 100 = 270.
  4. The exact answer is 268, so the estimate of 270 is 2 away. Close enough to catch a wrong answer with, and you did it in your head.

How it works.

  1. 01

    Do not work it out exactly. Round each number first. 371 is nearer 370 than 380, so to the nearest ten it becomes 370.

  2. 02

    103 is nearer 100 than 110, so to the nearest ten it becomes 100.

  3. 03

    Now it is easy enough to do in your head: 370 − 100 = 270.

  4. 04

    The exact answer is 268, so the estimate of 270 is 2 away. Close enough to catch a wrong answer with, and you did it in your head.

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