To answer this question - we need to know which pairs go together.
Knowing that guanine binds with cytosine and that adenine binds with thymine, we can work out the percentages.
28% G + 28% C = 54% GC (each guanine is paired with the a cytosine molecule and because each has to be paired, this amounts to the same percentage)
100% - 54% = 46% (now we have to subtract the percentage of GC pairs from the whole and we're left with 46%)
23% A + 23% T = 46% (this is again divided by two and we get our final answer)
So the answer in this case is 23% of thymine.