Because there are no boxes on the elevator, the maximum number of crate that can be on it will be the number which, when multiplicated by the average weight of a crate is equal to the maximum capacity of the elevator, which is 2,500 pounds in the present case.
From this, we are able to write as follows:

Where N stands for the maximum number of crates in the elevator.
But, because the number of crates can only be integer numbers, we will round it to N = 15, because 16 would already be over the maximum weight for the elevator.