If two printing machines work simultaneously at their respective constant rates

[Vincen]

If two printing machines work simultaneously at their respective constant rates, how much time (in minutes) will it take for them to print out 100 copies of a photograph?

I. One of the machines prints at thrice the constant rate of the other machine.
II. One of the machines prints at a constant rate of 15 prints per minute.

Answer: E

Source: e-GMAT