Final answer:
By applying Chargaff's rule that adenine pairs with thymine and cytosine pairs with guanine, and that these bases are present in equal quantities, we can deduce that if 20% of a DNA sample is thymine, then 30% of the sample must be cytosine.
Step-by-step explanation:
If 20% of a DNA sample is made up of thymine, T, then we can determine the percentage of cytosine, C, in the sample by understanding and applying Chargaff's rules. These rules state that DNA base pairing is highly specific: adenine (A) pairs with thymine (T), and cytosine (C) pairs with guanine (G). Therefore, the amount of adenine will equal the amount of thymine, and the amount of cytosine will equal the amount of guanine.
Given that there's 20% thymine in the DNA sample, it means that there's also 20% adenine, since A equals T. With this in mind and knowing that the total percentage of all four bases must add up to 100%, we can calculate that G and C together make up the remaining 60% of the DNA (100% - 20% A - 20% T = 60%). As C equals G, this 60% is equally divided between cytosine and guanine, giving us 30% for each. Hence, the percentage of the sample made up of cytosine, C, is 30%.