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Quick methods with 9, 19, 29

You learn to add and subtract numbers like 19, 29 and 99 by using the round number first.

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How to work it out

  1. Make the second number a round number: 19 becomes 20.
  2. Calculate with the round number.
  3. You added or took away too much: adjust that difference.

A worked example

46 + 19

  1. Make 19 into 20.
  2. Work it out: 46 + 20 = 66.
  3. You added 1 too many, so take it away: 66 − 1 = 65.
More examples

73 − 29

  1. Make 29 into 30.
  2. Work it out: 73 − 30 = 43.
  3. You took away 1 too many, so add it back: 43 + 1 = 44.

256 + 99

  1. Make 99 into 100.
  2. Work it out: 256 + 100 = 356.
  3. You added 1 too many, so take it away: 356 − 1 = 355.

58 + 18

  1. Make 18 into 20.
  2. Work it out: 58 + 20 = 78.
  3. You added 2 too many, so take them away: 78 − 2 = 76.

Try it yourself

  1. 512 − 99 =

  2. 63 + 49 =

Watch out

  • You adjust in the wrong direction: for 73 − 29 you do 43 − 1 = 42.

    You took away too much, so you must give some back. When you add, it is the other way round.

  • You forget to adjust: for 46 + 19 you say 66.

    After using the round number there is always one more step: adjust for the difference.

Tips

  • It also works with 18, 28 and 98: then you are 2 out.
  • For 99 the round number is 100, for 199 it is 200.

Why it works

19 is nearly 20, and 20 is much easier to work with. You use 1 too many, so you adjust for that afterwards. When you add, you take it away again; when you subtract, you add it back.