How does this do it?

Question

I can not figure out this site. It's quick to do it. and if has guessed my symbal every time. I'm totally stumped. You try and tell me. http://milaadesign.com/wizardy.html

Answer 1

It's algebra.

Any two digit number can be represented as 10x+y, where x is the first digit and y is the 2nd digit.

When you add the 2 digits, you get x+y.

When you subtract that from the original number, it becomes:

(10x+y) - (x+y) = 9x

So whatever number you end with is a multiple of 9.

If you noticed the symbols for all multiples of 9 were the 2 squiggly lines. You will always get that symbol.