Final answer:
In double-stranded DNA, if 30% of the bases are adenine, then the percentage of thymine will also be 30% due to complementary base pairing.
Step-by-step explanation:
In the structure of double-stranded DNA, the bases adenine (A) and thymine (T) are complementary base pairs, meaning that the percentage of adenine will always be equal to the percentage of thymine within the DNA molecule. Similarly, guanine (G) and cytosine (C) are complementary base pairs, with their percentages also being equal. Therefore, if 30% of the bases within a DNA molecule are adenine, then the percentage of thymine will also be 30%.