Whenever you write down a two-digit number, reverse the digits, and subtract one from the other, the result will always be divisible by 9.
fuckyeahmath:
Why: Any two-digit number can be written as 10T + U , where T is the tens digit and U is the units digit. Reversing the digits gives the number 10U + T, and subtracting the second from the first yields 10T+ U - (10T-U) = 9T-9 U=9(T-U), which is clearly divisible by 9.
