Final answer:
The percentage of cytosine in the DNA sample, where thymine is 15%, is 35% according to Chargaff's rules.
Step-by-step explanation:
If 15% of the nitrogenous bases in a sample of DNA from a particular organism is thymine, then according to Chargaff's rules, the percentage of adenine is also 15% since adenine (A) pairs with thymine (T) in DNA. Therefore, the amount of guanine (G) should equal the amount of cytosine (C) because these bases pair together as well. Since DNA has a double helix structure, and A pairs with T and G pairs with C, the total percentage of A and T together would be 30% (15% A + 15% T). Consequently, the percentage of G and C together must also be 70% to complete the 100% of the bases in the DNA strand. If G pairs with C equally, this means that both G and C would comprise half of this 70%, which indicates that cytosine would be 35% of the DNA bases (70% / 2).