An elevator goes up, at a rate of 750 feet per minute. is the height to which the elevator ascends proportional to the number of minutes it takes to get there

Yes. Proportions are pretty much “if X increases, Y also increases corresponding to X.”
In other words, the more time it ascends, the higher it will go. The greater height it ascends, the more time it took to get there.

If it were to ascend for 5 minutes, it would get to a greater height than if it were to ascend for only 3 minutes.