Adenine always pairs with thymine, cytosine with guanine and vice versa.
A=T; G=C
A=T=15%
A+T = 30%
We know that,
A+T+G+C = 100%
Then,
30+(G+C) = 100
G+C = 100-30
= 70
Since G=C,
C = 70/2
= 35
G = 70/2
= 35
Thus, there would be around 35% of cytosine in a deoxyribonucleic acid (DNA) molecule that has 15% of thymine.