Answer:15% cytosine
Explanation:We know that the sample is 35% adenine, which tells us that it is also 35% thymine. We know that cytosine and guanine pair together and will be present in equal amounts, so we can divide this final total by 2 to find our answer. The sample is 35% adenine, 35% thymine, 15% guanine, and 15% cytosine.
