Prove: if a rational number has a repeating decimal and the denominator is not divisible by 3 then the block of digits that repeats, taken as a decimal, is divisible by 9.

Examples:
7/13 = .538461 538461 538461 ...
538461 = 9*59829

15/22 = .6 81 81 81 ...
81 = 9*9