You give the initial condition as 0 kg salt in the tank at time = 0 (pure water)
If t = time and S(t) is the amount of salt in the tank as a function of time, then....
The equation that I get is S(t) = 236.8(1 - e^(t/740) )
That means when time goes to infinity, the amount of salt in is 236.8 kg.