In a room containing 20 000 ft3 of air, 600ft3 of fresh air flows in per minute,

Question:

In a room containing 20 000 ft3 of air, 600ft3 of fresh air flows in per minute, and the mixture (made practically uniform by circulating fans) is exhausted at a rate of 600 cubic feet per minute (cfm). What is the amount of fresh air y(t) at any time if y(0) = 0? After what time will 90% of the air be fresh?