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Digit Addition

Age 5 to 11 Challenge Level:

Here's a maths trick to try:
  1. Think of one of the numbers from 1 to 9.
  2. Add 9 to your number.
  3. Add the digits of your answer together. What is your new number?
What do you notice?

Try the trick again with a new starting number. What do you notice now?

Jonas tried the trick with a few different numbers. He said:


I think you will always get back to your original number, because adding 9 is the same as adding 10 and subtracting 1.


Do you agree with Jonas? Can you help him explain what's happening?

Can you explain the trick in a different way?