Explanation

According to Chargaff’s rule, in DNA, the proportion of adenine always equals to that of thymine and proportion of guanine always equal to that of cytosine, $i.e$., A=T and G=C. Thus, in a DNA, if guanine is 20%, cytosine also will be 20%. So, both adenine and thymine together will be 60%, $i.e.,$ 30% adenine and 30% thymine.