DNA consists of four nitrogenous bases: A, T, C, and G. Because A and T form a pair and C and G form a pair, the percentage composition of A is equal to T and the percentage composition of C is equal to G. This is called Chargaff's Rule.
In this sample, 20% is T. Because the percent composition of A is equal to T, we can expect 20% to also be A. That means that A and T make up 40% of the sample. That leaves 60% of the sample as C and G. The percentage composition of C must be equal to G, so divide 60% by 2 to get 30% G and 30% C.
So the answer is 30%.