What would you predict the %C (cytosine) to be based on this information?

The correct answer to this question is 36%. In a situation like this with a double-stranded DNA, we know G (guanine) is always complementary with C (cytosine) and A (adenine) is always complementary with T (thymine).

Therefore, the percentage of G has to equal the percentage of C, and the percentage of A has to equal the percentage of T. If G=14% then C=14%, totalling 28%. Therefore A+T = 72% (the remaining out of 100%), and we know A has to equal T so we can divide 72% in half to obtain the equal parts A (36%) and T (36%).