Chargaff's rules state that DNA from any cell of all organisms should have a 1:1 ratio (base Pair Rule) of pyrimidine and purine bases and, more specifically, that the amount of guanine is equal to cytosine and the amount of adenine is equal to thymine. This pattern is found in both strands of the DNA. %A = %T and %G = %C and A + T + G + C = 100% In the question, A is 15%. According to Chargaff's rule, T will also be 15%. A+T=30 Therefore G+C=100-30=70 According to Chargaff's rule C=G, Therefore, C=70/2=35, Hence G=35%