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Year 7 · Maths · Place value and ordering · Lesson 3

Estimating and checking

Estimate calculations by rounding to 1 significant figure, use estimates to check answers, and check with inverse operations.

Whiteboard video lesson: Estimating and checking

26-minute video lesson

Teaching, worked examples and 20 practice questions solved step by step.

Watch free for a month

What your child will learn

  • Round to 1 significant figure
  • Estimate × and ÷ calculations
  • Estimate + and − calculations
  • Check answers using estimates
  • Check answers using inverse operations

The method

  1. Round each number to 1 significant figure.
  2. Work out the easy calculation.
  3. Use ≈ for 'approximately equal to'.
  4. Check answers with an estimate or the inverse operation.

Key words: estimate, approximately, significant figure, inverse, check.

Worked examples

The first three practice questions from the lesson, with every step.

Question 1

Estimate 42 × 19.

  1. Step 1: Round to 1 s.f.: 42 ≈ 40, 19 ≈ 20
  2. Step 2: 40 × 20 = 800
  3. Step 3: (Exact: 798)

Answer: ≈ 800

Question 2

Estimate 68 × 31.

  1. Step 1: Round to 1 s.f.: 68 ≈ 70, 31 ≈ 30
  2. Step 2: 70 × 30 = 2100
  3. Step 3: (Exact: 2108)

Answer: ≈ 2100

Question 3

Estimate 395 ÷ 19.

  1. Step 1: Round to 1 s.f.: 395 ≈ 400, 19 ≈ 20
  2. Step 2: 400 ÷ 20 = 20
  3. Step 3: (Exact: 20.79)

Answer: ≈ 20