26% of the DNA molecule will be adenine because, cytosine is always paired with guanine. Because of this, there will be equal parts of cytosine and guanine. In total, 48% of the DNA molecule will be either cytosine or guanine. If 48% of the molecule is cytosine or guanine, then 52% of it will be either adenine or thymine (which are also only ever paired with each other). Therefore, you divide 52 in half - because there will be equal parts of adenine and thymine - and get 26% of each.
