This is one of those problems that makes more sense with context. The teacher had the students working on "reasonableness", which is essentially "does the question I'm asking make sense?". The students were probably instructed to ignore actually trying to solve the problem when presented with one, but instead explain why the question either does or doesn't make sense.
In this case the student potentially misunderstood the task. The failure on the teacher's part is wording the question in such a way that it actually has a reasonable solution, and isn't necessarily an unreasonable question.
