Final answer:
By applying Chargaff's rules, which state that the percentage of adenine equals thymine and guanine equals cytosine, we calculate the percentage of cytosine to be 35% when 15% of the bases are adenine.
Step-by-step explanation:
If 15% of the bases in a DNA molecule are adenine, we can determine the percentage of cytosine by applying Chargaff's rules. According to these rules, adenine (A) pairs with thymine (T), and guanine (G) pairs with cytosine (C). Therefore, the percentage of adenine will always equal the percentage of thymine, and the percentage of guanine will always equal the percentage of cytosine.
Since 15% of the bases are adenine, there will also be 15% thymine. This accounts for 30% of the bases, leaving 70% for guanine and cytosine combined. Because guanine and cytosine are present in equal amounts, we divide the remaining 70% by two to get the percentage for cytosine, which is 35%.