Final answer:
According to Chargaff's rule, if adenine is 30%, then thymine is also 30%, and guanine and cytosine would each be 20%.
Step-by-step explanation:
Based on Chargaff's rule, the amount of adenine (A) in DNA is always equal to the amount of thymine (T), and the amount of cytosine (C) is always equal to the amount of guanine (G).
This means that if adenine is 30%, then thymine must also be 30%. Since the total percentage of bases must equal 100%, we can subtract the percentages of adenine and thymine from 100% to find the percentages of cytosine and guanine. Therefore, guanine and cytosine would each be 20%.