Answer
13 adenine residues and 0 uracil residues.
Work Step by Step
The segment of DNA is 20 base pairs long, which means there are 40 residues. If 7 of the residues are guanine, this must mean that there are also 7 cytosine residues. So, 14 of the 40 residues are G and C. Subtracting this from 40, the 26 remaining residues must be equal parts adenine and thymine. Therefore, the answer is 13 adenine residues. There are no uracil residues because the question deals with DNA, not RNA.
