Haven't you already failed the aptitude test by having to ask?
(no disrespect intended)
More seriously, I suspect the question could be read in different ways and lead to different answers.
... but perhaps this is where aptitude comes in, realising that the question can be read more than one way.
Maybe it is HOW you answer that shows aptitude, more than giving one of several possible right answers?
... or maybe stating your assumptions would show aptitude, since there is one obvious way to interpret the question, and one or more less obvious interpretations.
...or maybe the correct answer would be "can you clarify the question please?".
...or maybe I do not understand?
... or maybe I am thinking too deep, and the person testing your aptitude is simply looking for the one obvious answer to the obvious interpretation.
in which case... start by thinking of the chances of one person entering the room within a particular 10 minute period. Think the logic through from there.