In the DNA molecule, the number of nucleotides with guanine is 20% of the total. The proportion of nucleotides with thymine in this molecule is

According to the rule of Chargaff (based on complementarity)
The amount of adenine is equal to the amount of thymine, and guanine to cytosine: A = T, G = С.
The number of purines is equal to the number of pyrimidines: A + G = T + C.
As a result: (A + G) + (T + C) = 100%

Decision.
The amount of guanine is equal to the amount of cytosine, which means = 20%, 60% remains for thymine and adenine, they are also the same amount, which means 30%.