Since I am not a scientist, I can neither confirm nor deny the previous answer. However, I would plug the numbers into the equation myself to double check the work and see if that equation comes up when searching for this kind of problem.
I wouldn’t know where to begin with this, so the only piece of advice I can offer is this: finding the requirement for one person and then multiplying that by twenty, as the previous answer suggests, sounds like a good course of action to me.
It’s also probably the easiest way to do it, since this particular problem offers all the statistics needed to do it this way. We are given the time it takes, how far the elevator has to go, and how much an average passenger wieghs.