Because we learn over time how things work and how they don't work.
Theoretically it is the Chargaff's rule which entails that the ratio of purines and pyrimidines are 1. So following that rule you can solve this question.
G (purine) = C (pyrimidine) and
A (purine) = T (pyrimidine)
So G + C would be 28% and hence
A +C would be 72% i.e. 36% each.
Please note that their are certain modified bases besides these four are also there including methyl Cytosine, Methylated Adenine. But htese modification does not violate the Chargaff's rule and they follow standard Watson Crick base pairing.
