When decoding the maize genome, it was found that the fraction of nucleotides with cytosine in a fragment of a DNA

When decoding the maize genome, it was found that the fraction of nucleotides with cytosine in a fragment of a DNA molecule is 30%. Using Chargaff’s rule, which describes the quantitative ratios between different types of nitrogenous bases in DNA (G + T = A + C), calculate the amount (in%) in this sample of nucleotides with adenine.

Adenine (A) is complementary to thymine (T), and guanine (G) is complementary to cytosine (C), therefore the number of complementary nucleotides is the same; Chargaff’s rule: (A + T) + (G + C) = 100%
By condition C = 30%
A + T = 100% – (30% + 30%) = 40%
A = T = 40%: 2 = 20%