According to Chargaff’s rule: Guanine pairs with cytosine. It Guanine is 40%, then cytosine will also be 40%. Similarly, Adenine pairs with thymine, if guanine is 40%, the remaining 60% will be Adenine. So Thymine will also be 60%.
Thus, A : T = G : C = 1
and 60 : 60 = 40 : 40 = 1.