A tank initially contains 60 gal of pure water. Brine containing 1 lb of salt per gallon enters the tank at 2 gal/min, and the (perfectly mixed) solution leaves the tank at 3 gal/min; thus the tank is empty after exactly 1 h.

(a) Find the amount of salt in the tank after t minutes.

(b) What is the maximum amount of salt ever in the tank?

My working:

Let x(t) (in lb) be the amount of salt in the tank at time t.

(a)

Multiplying by , we get

Solved it! Don't bother.