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

Then (rate of saltin) - (rate of saltout).

Rate in = (concentration of salt in inflow)(rate of inflow) = (1 lb/gal)(3 gal/min) = 3 lb/min.

Rate out = (concentration of salt in tank at time t)(rate of outflow)

= (3 gal/min) = .

Therefore the differential equation you need to solve is

subject to the boundary condition x(0) = 20.