We use Chargaff's rule to get the answer.
[A] + [G] = [C]+ [T].
[A] + [G] + [C] + [T] = 100%
Where is A is Adenine, G is Gaunine, T is Thymine and C is Cytosine.
In DNA, Adenine always pairs with Thymine, and Guanine always pairs with Cytosine.
Therefore if Thymine is 35%, then Adenine will also be 35% to make 70% in total.
The remaining percentage will be 100% - 70% = 30%.
The 30% will be shared equally among Cytosine and Guanine, at 15% each. Therefore Cytosine will be 15%
