Final answer:
Using Chargaff's rules, if a DNA strand contains 20% adenine (A), it must also contain 20% thymine (T), and the remaining 60% of the bases would be guanine (G) and cytosine (C) in equal amounts, resulting in 30% cytosine (C).
Step-by-step explanation:
The question relates to the composition of DNA and uses Chargaff's rules, which state that in a DNA molecule, 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). Since you've stated that the DNA strand contains 20% adenine (A), according to Chargaff's rules, it would also contain 20% thymine (T) because A pairs with T.
Now, knowing that the total amount of adenine (A) and thymine (T) together is 40% (since each is 20%), and that the total percentage of DNA bases must add up to 100%, we can deduce that the remaining 60% must be divided equally between guanine (G) and cytosine (C), as G pairs with C. Therefore, the DNA strand contains 30% cytosine (C) because 60% divided by 2 is 30%.