There are x people in a survey.
y% of people checked box A
z% of people checked box B
It is possible to check BOTH boxes
What percentage of x people checked both boxes?
Am I correct in saying that this question is unanswerable without more information?
The probability of finding box $A$ checked is $y$. The probability of finding box $B$ checked is $z$. If whether or not a person checks $A$ is independent of whether or not a person checks $B$, then the probability of finding both $A$ and $B$ checked is $x=y(z)$. If those two events are in fact dependent, then the answer is not so clear... you would need more information.