A 400-gallon tank initially contains 200 gallons of water containing 2 parts per billion by weight of dioxin, an extremely potent carcinogen. Suppose water containing 5 parts per billion flows into the top of the tank at a rate of 4 gallons per minute. The water in the tank is kept well mixed, and 2 gallons per minute are removed from the bottom of the tank. How much dioxin is in the tank when the tank is full?
I'm going to use D(t) as the amount of dioxin.
The Attempt at a Solution
dD/dt = (5)(4) - 2*(D(t)/(200+2t))
Using an integrating factor of t + 100, and the initial condition of D(0) = 2, I got that D(t) = (10t2 + 2000t + 200)/(t+100)
But when using t = 100, I get the answer to be 1501 ppb instead of the more appropriate 4.25 ppb. Where did I go wrong?