If 30% of Adenine is present, it would be reasonable to assume that 30% of Thymine will be present, as the ratio between the two complementary nitrogenous bases will be roughly equal.
Therefore, that leaves us with 40% between Cytosine and Guanine. Since the ratio between them would be equal, it gives us 20% of Cytosine and 20% of Guanine.
Therefore:
30% of A + 30% of T + 20% of C + 20% of G = 100% DNA
