Diff. Eq. Tank Problem

Question

A very large tank initially contains 100L of pure water. Starting at time t=0 a solution with a salt concentration of 0.5kg/L is added at a rate of 6L/min. The solution is kept thoroughly mixed and is drained from the tank at a rate of 4L/min. How much kg of salt is in the tank after 30 minutes?

Answer 1

I now see that I stopped one step short of the answer. A(30)=((30)^3+150(30)^2+7500(30))/(30)^2

=(27000+135000+225000)/900

=387000/900

=430

430 kg of salt