If a dna molecule is found to be composed of 35% thymine, what percentage of cytosine would be expected, and why?

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%