Well, if you put it that way... I suppose only thirteen people will be able to make it off the boat. Five on the first raft, four on the second and third (unfortunately, someone needs to stay on to row it back).

And as for the lazy student? If he's devious about his teacher's exact wording - and willing to do schoolwork in the late hours of the night - he'll be able to make it out with just eight hours of study.