According to Erwin Chargaff's observation for double-stranded DNA, the ratios between Adenine and Thymine and between Guanine and Cytosine are constant and equal one. Therefore, the percentage of Adenine (A) equals the percentage of Thymine (T), and the percentage of Guanine (G) equals the percentage of Cytosine (C). If A = 30%, then T = 30%. The remaining 40% (100% - 60%) is split equally between G and C, so G = 20% and C = 20%. ,