A 500 gallon tank is full of a solution containing 25 pounds of concentrate. Starting at time
t = 0, distilled water is admitted to the tank at a rate of 15 gallons per minute, and the well-stirred solution is withdrawn at the same rate.
(i) Find the amount of concentrate
Q in the solution as a function of
t.
(ii) Find the time at which the amount of concentrate in the tank reaches 15 pounds.
(iii) Find the quantity of the concentrate in the solution as

.