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%