If each of the a,b,c,d is a decimal digit - for how many ordered quadruplets (a,b,c,d) is the sum S= a+b+6c+d divisible by 3 ?

Answer: within 20 sec.

I agree with Charlie's interpretation of the question, and his solution.  It doesn't seem like a 20 sec question to me, though. 

If each of a,b,c,d is a digit from 1 to 9 instead of 0 to 9, then it can be solved remarkably quickly - I wonder if that was his intent?

Posted by tomarken on 2014-04-09 12:57:20