If the half-life of fluorine-18 is 110 minutes, then approximately what percentage of a sample will remain...

after 200 minutes?
A. 28%
B. 13%
C. 50%
D.25%

After 110 minutes, 50% will be left.
After 220 minutes, 25% will be left.

However, you want it after 200 minutes. Because 25% is left after 220 minutes, after just 200 there will be a bit more than 25% left.

The only answer that fits this is 28%. It can't be 25% or less than 25%, and we already worked out that 50% would be left after just 110 minutes.

So the answer is A, 28%.