dS/dt = salt in - salt out

dS/dt = (concentration)(flow rate in) - (concentration)(flow rate out)

dS/dt = (20g/l)(3l/min) - (Sg/120l)(3l/min)

dS/dt = 60 - 1/40S = 1/40( 2400 -S)

Separate and solve note equillibrium solution is S =2400 since S(0) = 400

2400 - S > 0