An organism's genome is analyzed and found to contain 18% guanine. What percentage of that organism's DNA is adenine?

32%

Explanation:

According to Chargaff in his complementary base pairing rule, he stated that the proportion of purine and pyrimidine bases is equal to 100%

That is, A + T + G + C = 100%

Where A=T and G=C i. e the proportion of each base pair is equal.

Hence, if the Guanine content is 18%, it means the Cytosine content of that DNA molecule must also be 18%. Therefore:

18 + 18 + A + T = 100

A + T = 100 - 36

A + T = 64

If the sum proportion of Adenine and Thymine is 64% and each must contain equal proportions, A or T = 64/2 = 32

Hence, Adenine contains 32%, likewise Thymine.