If a nucleus has 20% Thymine, how much Guanine does it have? Explanation please?

Answer 1

If we're talking 20% of the DNA in a nucleus here is Thymine, then we assume that 20% of the DNA is Adenine, since A's pair with T's. That leaves 60% to be divided equally amongst Guanine and Cytosine (G's pair with C's), leaving 30% for each of them.
A% + T% + C% + G% = 100%

Answer 2

In percentages

A = T
G = C

A+T+C+G = 100%

T = 20% and A = 20%
G+C = 60%

G = 30%

Answer 3

30%