There are 4 bases: Cytosine, Guanine, Adenine, and Thymine(sometimes Uracil). Cytosine pairs with Guanine and Adenine with Thymine. If there's 20% Cytosine then there must be an equal amount of Guanine for the pairs to match up. 40% of the bases are either Cytosine or Guanine then 60% is Adenine or Thymine. Equal amounts of each so divide by 2 and 30% is expected to be Thymine.
