There are four bases found in DNA: adenine (A), cytosine (C), guanine (G), and thymine (T). Adenine forms a base pair with thymine, and cytosine forms a base pair with guanine. There is a one-to-one relationship in these base pairings (Chargaff’s rule), which means that if you know the percentage of any one of them within a given DNA sample, you can calculate the percentages of the other three. In this case, you're given the percentage of guanine, and you want to find out the percentage of adenine.
Since guanine base-pairs with cytosine and since there must be as much cytosine as there is guanine, 41% of the bases in this gene are cytosine as well. That means that adenine and thymine <em>together </em>make up the remaining 18% (100% − 41% G − 41% C) of the base pairs. If there must be an equivalence in the number of thymine and adenine bases per Chargaff's rule, then half of the remaining base pairs must comprise adenine and the other half comprise thymine. Half of 18% is 9%.
Thus, adenine makes up 9% of the bases in this gene.
