Adenine (purine) is the same as Thymine (pyrimidine); i.e., A = T or A/T = 1.
Cytosine (pyrimidine) is equal to guanine (purine); in other words, C = G, or C/G = 1.
The total number of bases of purine equals the total number of bases of pyrimidine, i.e. A + G = C + T