Math puzzles to beef up your brain

Today I accidentally came across a few interesting math puzzles and tried to solve them. Here's one which put me in trouble.

"The following is what seems to be a mathematical proof that ten equals 9.999999.... What's wrong with it?

               a =  9.999999...
            10a = 99.999999...
        10a - a = 90
             9a = 90
              a = 10 "

You can find more puzzles here - many of them are straightforward though.
Give it a try. Good luck!