1. The problem statement, all variables and given/known data Prove the following a is congruent to s(mod 9) 2. Relevant equations a=drdr-1***d1d0 a=d0+ d110+ d2102+...+dr10r s= d0+ d1 +...+dr 3. The attempt at a solution we know that 10-1=9 so we can say that 10 is congruent to 1(mod 9) so we know that a is congruent to d0+ d110+ d2102+...+dr10r which is congruent to d0+ d1+ d2+...+dr (mod 9). Is this a step in the right direction? Does anything else need to be shown?