What is the answer to this problem? Would you expect the relationship between the number of 50-meter laps Trevor swims and the time it takes him to swim those laps to be proportional?

The time it takes him to swim some laps is proportional to the number of laps ... for a while, as long as he keeps up the same speed. But later, as he wears out and begins to swim slower and slower until finally he slips beneath the waves and sinks like a rock, the direct proportionality breaks down.

That's what I would expect. Other reasonable people may disagree.