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MathidiaMathematics Academy
Place value and rounding
Lesson 5 of 75
Review

Estimate and check

Objective. Use rounding to estimate calculations and test whether answers are sensible.

  1. 1. Learn
  2. 2. Worked example
  3. 3. Practice
  4. 4. Feedback
  5. 5. Continue

Learn: the key idea

Estimating means rounding each number to an easy value, often one significant figure, and calculating with those. It gives a quick idea of the size of the answer so you can spot mistakes. For example, 4 879 + 2 103 ≈ 5 000 + 2 000 = 7 000, so an answer of 69 820 must be wrong. Use ≈ to show a value is an estimate, then compare it with your exact answer.

Worked example

Estimate 4 879 + 2 103 by rounding each number to one significant figure. Is 69 820 a sensible exact answer?

-132592504416318221013+2
Number line from -132 to 1013 with a jump of 2 forwards from 879
  1. 1Round 4 879 to one significant figure: 5 000.
  2. 2Round 2 103 to one significant figure: 2 000.
  3. 3Estimate: 5 000 + 2 000 = 7 000.
  4. 469 820 is about ten times too big, so it is not sensible (the exact answer is 6 982).

Answer: ≈ 7 000; no

Concept mastery

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Practice

Practice happens on its own screen, one question at a time. Answers stay hidden until you submit yours, and you can stop and pick up at the same question later.

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Common mistake. Pupils often round to awkward values or calculate exactly anyway, which defeats the purpose. Round each number to one significant figure, use the ≈ sign, and check the exact answer is close.

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