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Addition & Subtraction Tricks

Adding 9 Quickly – Add 10, Take Away 1

Adding 9 is awkward, but adding 10 is easy: just fix it afterwards.

This page explains the rule, shows why it works, and walks through worked examples. Then you can try it yourself and check your understanding with practice questions. Use the previous and next links at the bottom to move through the tricks in order.

The rule

The Rule: To add 9 to a number, add 10 and then subtract 1.

Why it works

Why It Works: Adding exactly 9 is a little fiddly, but adding 10 is instant — you just raise the tens digit by one. Since 9 is only 1 less than 10, adding 10 and then stepping back down by 1 lands you on precisely the right answer, with far less mental effort than adding 9 directly.

Formal Proof (for advanced learners)

For any n: n + 9 = n + 10 − 1, a direct rearrangement of the addition — adding 10 and subtracting 1 always gives the same result as adding 9.

Examples

  • 56 + 9 → 56 + 10 = 66, then 66 − 1 = 65.
  • 28 + 9 → 38 − 1 = 37.
  • 145 + 9 → 155 − 1 = 154.

Step by step

Example A: 485 + 9

  1. Add 10: 485 + 10 = 495.
  2. Take away 1: 495 − 1 = 494.

Example B: 1996 + 9

  1. Add 10: 1996 + 10 = 2006.
  2. Take away 1: 2006 − 1 = 2005.

Try it yourself

Now it is your turn. A new random question appears each time. Type your answer in the box and press Check Answer. If you are right, we show how the trick got you there, step by step. If you are not, you will see the full working two ways, the shortcut and the usual method, so you can spot exactly where your answer differed. Press New Question as often as you like: a few quick rounds is the best way to make the trick feel automatic.

Try It Yourself

Practice questions

Have a go on paper first, then tap Show answer to check your method.

  1. Work out 34 + 9.

    Show answer

    43. Add 10: 34 + 10 = 44. Take away 1: 44 − 1 = 43.

  2. Work out 58 + 9.

    Show answer

    67. Add 10: 58 + 10 = 68. Take away 1: 68 − 1 = 67.

  3. Work out 76 + 9.

    Show answer

    85. Add 10: 76 + 10 = 86. Take away 1: 86 − 1 = 85.

  4. Work out 197 + 9.

    Show answer

    206. Add 10: 197 + 10 = 207. Take away 1: 207 − 1 = 206.

  5. You have 163 points and win a 9-point bonus. How many points do you have now?

    Show answer

    172. Add 10: 163 + 10 = 173. Take away 1: 173 − 1 = 172.

Tips and watch-outs

  • The same idea works for 19 (add 20, take 1) and 99 (add 100, take 1).
  • To subtract 9, do the reverse: subtract 10 and add 1 (56 − 9 = 46 + 1 = 47).