Module 2 Knowing and using number facts
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Can I explain how I use approximations to help estimate the answer to a calculation?

Example review questions

  • Apples weigh about 190 g each. How many apples would you expect to get in a 2 kg bag?
  • Is 4, 5 or 6 the best estimate for the answer to 44.81 ÷ 8.92? Explain how you decided.
  • I buy six books that cost £7.99 each and four CDs that cost £12.99 each. Use approximation to work out the total cost to the nearest pound.
  • Estimate the answers to these calculations: 2593 + 6278, 2605 − 1997, 245 × 19, 786 ÷ 38 Explain your methods.
  • A lawn is 19.5 m long and 4.5 m wide. Is its area greater or less than 100 square metres? Explain how you know.

Teaching guidance

This teaching guidance document suggests some of the key vocabulary, models, images and practical equipment that children should experience and be able to use. It also includes some teaching tips to provide a few starting points for ways of supporting children with this area of mathematics.

PDF file Teaching guidance: Can I explain how I use approximation to help estimate the answer to a calculation? | 44KB new window

Consolidation and practice

These resources are to support children in guided or independent work.

PDF file Number boards guidance | 198KB new window

XLS file Number boards spreadsheet | 92KB new window

PDF file Rounding guidance | 139KB new window

XLS file Rounding decimals spreadsheet | 40KB new window

PDF file Make numbers guidance | 154KB new window

XLS file Make numbers spreadsheet | 135KB new window

DOC file Calculator activities | 128KB new window

DOC file Matching cards approximation activity | 35KB new window

Visit the nrich website new window for further activities to support practice and application.

Opportunities to use and apply

Possible contexts include:

  • Word problems, e.g. Roughly how many pot plants can I buy with £50 if each plant costs £2.99?
  • Checking calculations, e.g. Emma is trying to work out 29.5 x 7.8 and her calculator displays 183.3. Do you think this is correct? Explain your answer.
  • Missing number calculations, e.g. Use approximation to find the missing digit: empty box1.2 x 18.9 = 778.68.
  • Games. e.g. Call my calculation bluff – children choose an answer to a calculation from three given numbers.
  • Measures problems, e.g. If drawing pins weigh 4.2 g each, roughly how many drawing pins would you expect to have in a 250g box?

 

Confirming learning

Ask probing questions such as:

  • A car park has 76 rows for parking. There are 52 car spaces in each row. Which of these is the best way to estimate how many cars can park altogether? 80 × 60 = 4800, 80 × 50 = 4000, 70 × 60 = 4200. Explain your choice.
  • Which of these two numbers multiplied together give the product closest to 24? 7.9, 9.2, 2.1, 2.8.
  • Without working out each calculation exactly, predict which will have answers between 40 and 60: 6.8 × 7.5, 80.03 − 27.2, 997 ÷ 19.
  • Circle the closest estimate for the answer to 72.34 ÷ 8.91. Choose from 6, 7, 8, 9, 10, 11. How did you decide?
  • Use approximation to predict the missing digits in this calculation: empty box1.2 × 1empty box.9 = 778.68.