Final answer:
According to Chargaff's rules, if a DNA sample has 15% Adenine, it also has 15% Thymine. After subtracting the percentage of Adenine and Thymine from 100%, the remaining percentage for Guanine and Cytosine combined is 70%. As these are in equal amounts, Cytosine also makes up 35% of the DNA.
Step-by-step explanation:
If a sample of DNA has 15% Adenine, according to Chargaff's rules, we know that the amount of Adenine (A) is always equal to the amount of Thymine (T), and the amount of Guanine (G) is equal to the amount of Cytosine (C). Therefore, if there is 15% adenine, there must also be 15% thymine due to base pairing.
Since DNA is made up of a total of 100%, we can subtract the combined percentage of adenine and thymine (15% + 15% = 30%) from 100% to get the combined percentage of guanine and cytosine, which is 70%. Because guanine and cytosine are present in equal amounts, we divide this number by two to get the percentage of cytosine. Thus, the percentage of cytosine in the sample is 35%.