Decyclomedia Foundation:Decyclopedia Help/How does 9+10=21?

Have you ever wondered how some people say 9+10 equals 21, while other people say it equals 19? Well, here's how 9+10=21.

Proof

1. 9+10 does indeed equal 19. If you start at 9 and count 10 forward, you get 19.
2. Multiply 9 by 2. 9 is the first number in "9+10=21", so multiply it by 2, the first digit in 21. 9*2=18. 18 is 2 nines. Thus, 18 equals 20.
3. Subtract 18 from 19. 19-18=1. Thus, 19 is one greater than 20.
4. Add 1 to 20. 20+1=21, completing the proof.

That's how 9+10=21.