If 22% of a DNA sample is made up of cytosine, C, what percentage of the sample is made up of adenine, A?

30%

Explanation:

Together, that is 40 percent of the total sample. You can subtract that 40 percent from 100 percent and determine 60 percent of the sample must be adenine and thymine together. Since those two bases always exist in equal concentrations, you know the DNA sample is 30 percent adenine.

28% of Adenine

Explanation:

Chargaff's Rule states that A = T and C = G

Because of that, the sample must have the equation:

A + C = T + G

G must have the same amount as C so,

A + .22 = T + .22

Both sides of the equation must equal each other by .50, so .50 - .22 = .28

.28 x 100 = 28%