So Adenine and Thymine are base pairs so they have to be equal to each other. So Adenine and Thymine are both 15% or 30% together.
This means there is still 70% left.
The next base pair is Cytosine and Guanine. they also have to equal to each other. Since there is 70% left you divide that number by two and you get 35%.
This means 35% is Cytosine in the strand of DNA
