Then (rate of salt in) - (rate of salt out).
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.