Final answer:
According to Chargaff's rules, if a DNA strand contains 20% adenine, it will also contain 20% thymine because adenine and thymine pair together. The remaining 60% is split equally between cytosine and guanine, resulting in 30% each.
Step-by-step explanation:
If a strand of DNA has 20% of its bases as adenine (A), then according to Chargaff's rules, we can assert that 20% of the bases will also be thymine (T), since in DNA, the amount of adenine equals the amount of thymine (A = T). Given that adenine and thymine are complementary bases that pair together in the DNA double helix, the same proportion must hold true.
To figure out how much cytosine (C) and guanine (G) there is, we can consider that the total percentage of the four bases (A, T, C, G) must add up to 100%. With adenine and thymine together accounting for 40% (20% each), the remaining 60% must be split equally between cytosine and guanine, because Chargaff's rules also state that the amount of guanine equals the amount of cytosine (G = C). Therefore, there would be 30% cytosine and 30% guanine in the DNA strand.