Try this without reading the answers first:

1. Pick your favorite number from 1 to 9
2. Multiply it with 3
3. Add 3 and multiply it with 3, again
4. You get a number
5. You will get a two or three digit nuber.. sum the digits. (eg. 527 = 5 + 2 + 7 = 14)
6. And now.. check the answer!

Click on show to see the results

show