If the percentage of thymine is 30% what would the percentage of cytosine be

We use Chargaff's rule.

[A] + [G] = [C] + [T]

[A] + [G] + [C] + [T] = 100%

A is Adenine, G is Guanine, C is cytosine, T is Thymine.

Adenine always pairs with Thymine.

Guanine always pairs with Cytosine

So if Thymine is 30% then Adenine is 30% → 60% total

Therefore 100% - 60% = 40%

40% is shared equally among Cytosine and Guanine

Therefore Cytosine will be 20% and Guanine 20%

Cytosine is 20%