Final answer:
According to Chargaff's rules, the percentage of adenine in a bacterial genome is equal to that of thymine. Given that cytosine is 20%, adenine would also be 30% as the percentages of adenine and thymine must be equal, accounting for 60% of the genome together.
Step-by-step explanation:
The percentage of cytosine (C) in a bacterial genome is given as 20%. According to Chargaff's rules, which state that in DNA, the amount of adenine (A) is equal to the amount of thymine (T) and the amount of guanine (G) is equal to the amount of cytosine (C), the percentage of adenine can be determined. Since C is 20%, G would also be 20% due to their pairing. This leaves 60% for A and T combined, and because A equals T, A must be 30% of the genome, just as T is 30%. This is a direct application of Chargaff's base pairing rules, which provides a quick method to determine the relative percentages of the bases when one of them is known.