Skip to main content
### Number and algebra

### Geometry and measure

### Probability and statistics

### Working mathematically

### For younger learners

### Advanced mathematics

# Digit Addition

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

Or search by topic

Age 5 to 11

Challenge Level

*Digit Addition printable sheet*

Here's a maths trick to try:

- Think of one of the numbers from 1 to 9.
- Add 9 to your number.
- 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 from 1 to 9. What do you notice now?

**Will this always happen if you start with a number from 1 to 9?**

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 is happening?
Can you explain the trick in a different way?**

*If you enjoyed this problem, you may like to take a look at The Number Jumbler.*