22% cytosine, because each pair has an equal amount of complementary base percentages. For example, if you are given 20% Adenine, then there has to be 20% Thymine as well, and you can also calculate cytosine and guanine by looking at the remaining percentage (80%). Because 80% is left over and each complementary base pair has equal amounts of base percentages, Cytosine and Guanine would each have 40%.
