Guanine and Cytosine are complementary pairs in the DNA molecule (which means that Cytosine is bonded via hydrogen bonds to Guanine). This is the same for Adenine and Thymine. As such if a sample of DNA contains 20% Cytosine then it also contains approximately 20% Guanine.
The percentage of Thymine + Adenine would be 100% - 40% = 60%... Since this 60% should be equally shared between Thymine and Adenine then it implies that the percentage Adenine would be approximately 30%.
