Final answer:
Using Chargaff's rules, if cytosine is 18% of a DNA sample, the likely percentage of adenine is 32% because adenine pairs with thymine and cytosine pairs with guanine, achieving a balance in the DNA structure.
Step-by-step explanation:
If the percentage of cytosine in a sample of DNA is 18%, according to Chargaff's rules, which state that the amount of adenine (A) equals the amount of thymine (T), and the amount of guanine (G) equals the amount of cytosine (C), we can deduce the percentage of the other bases. Since cytosine is at 18%, guanine will also be 18%. Because A pairs with T and C pairs with G, if we have 18% cytosine and 18% guanine, their combined percentage is 36%. As the total amount of bases should add up to 100%, this leaves us with 64% for adenine and thymine together. Hence, the likely percentage of adenine will be 32% to complement the 32% of thymine.