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