A sample of DNA has 10% Guanine. What is the percentage of Thymine in that sample?

Guanine (G) in DNA is one-to-one coupled to cytosine (C), so that also makes up for 10%.

Then there's 80% left for the other one-to-one couple: Thymine (T) and adenine (A). So 40% T and 40% A.