hi docasrpa! here's the answer.
in DNA, there is a principle "base pairing rule" as we all know. Adenine always pairs with Thymine, and Guanine pairs with Cytosine. So, to follow this rule, the molar conc. of [A] = [T] and [G] = [C]. So if [A] = 22%, [A+T] = 44% and [G+C] =100 - 44 = 56%. so [G]=[C]=56/2=28%
hope this help. cheers😀