differential equation 2

A conference room contains 3 000 cubic meters of air which is free of carbon monoxide. The ventilation system blows in air free of carbon monoxide, at a rate of 0.3 cubic meters per minute and extracts it at the same rate. If at t = 0, people in the room start smoking and add carbon monoxide to the room at a rate of 0.02 cubic meters per second, how long before the air in the room contains 0.015% carbon monoxide?


The answer is 22.5254 seconds.
But how to work out the answer??