Let be the amount of fertilizer in the tank at time . Then
This ODE is separable, and you can write
Integrating both sides gives
At , there is no fertilizer in the tank, so . This means
so that the amount of fertilizer in the tank is given by
This function is constantly increasing, so it never achieves its maximum, but there is a strict upper bound as . You have lbs of fertilizer in the tank, which makes sense. Given enough time, the tank will be completely saturated with the fertilizer solution, so 2 lb/gal*100 gal = 200 lbs.