The time (t) required to empty a tank varies inversely as the rate (r) of pumping. A pump can empty a tank in 90 minutes at the rate of 1200 L/min. How long will the pump take to empty the tank at 3000 L/min?
90 minutes at 1200 L/min means that the tank holds
To empty the tank at a rate of 3000 L/m will take the time of
Observe that this is exactly the same as in first principles.