A tank contains 25 L of water. A solution with a salt concentration of 0.4 kg/L is added at a rate of 5 L/min. The solution is kept mixed and is drained from the tank at a rate of 3 L/min. If y(t)
is the amount of salt (in kilograms) after t minutes, show that y satisfies the differential equation
dy
dt
= 2 −
3y
25 + 2t
.
Solve this equation and find the concentration after 20 minutes. (Round your answer to four decimal places.)