If a strand of DNA is 7% Adenine, what percent Cytosine is it?

43 % cytosine

Explanation:

The complementary base pairs in DNA are A-T and C-G.

Thus, if the strand is 7 % A, it is also 7 % T.

If 14 % of DNA is A-T base pairs, it is 86 % C-G.

Half of the 86 % is C, so the DNA is

43 % cytosine