Rpm means number of rotations per minute.
rps means number of rotations per second.
rpm = 60 * rps
rps = rpm / 60
In one second, an object rotates 20 times, or, completes 20 rotations (rps). So in 60 seconds, it will complete 60* 20 = 1200 rotations. So rpm value is 1200.
In one minute (60 seconds) if an object makes 2400 rotations (=rpm) , it means that in one second it will rotate 2400/60 = 40 rotations (rps).