All the thymine binds with adenine, therefore there is the same number of nucleotides that contain thymine as there are of adenine
If we have 20% thymine bases there will certainly be 20% adenine
we are left with 60% that are guanine and cytosine, we know that all cytosine binds with guanine, therefore there is the same number of cytosine and guanine bases. 2G=60 G=60/2=30% guanine bases