According to Chargaff’s rule all cellular DNAs, regardless of the species, number of adenosine residues is equal to number of thymidine residues which means that A=T; and the number of guanosine residues is equal to the number of cytidine residues; G= C. If there is 20% cytosine which means guanine percentage is also 20%. Thus, the percent of A+T= 100-(G+C)A+T = 100-40 =60,  Adenine percent= 60/2= 30.