learnbits.app

Choosing a handy method

You learn to choose the handiest method before you start calculating.

Practise thisWhiteboardMake it a quiz

How to work it out

  1. Look at the numbers before you start.
  2. Just under a round number, like 199? Round and adjust.
  3. Two close numbers in a take-away? Count on.
  4. An even number times a number you can double? Halve and double.

Which sum is easy to work out by rounding and adjusting? 546 + 199, 546 + 234, 546 + 40

  1. Look at the second number.
  2. 199 is just under 200.
  3. Add 200 and take 1 away: 746 − 1 = 745.
  4. So 546 + 199.
More examples

Which sum is easy to work out by counting on? 405 − 397, 405 − 130, 405 − 50

397 and 405 are close together: from 397 to 405 is 8.

Which sum is easy to work out by halving and doubling? 4 × 35, 5 × 37, 3 × 36

4 × 35 = 2 × 70 = 140.