Differential equation problem...?

this is a " change is salt {S} = incoming salt - outgoing salt" problem ...incoming is [0.1][2] and outgoing is [S/1000][2]...thus dS/dt = 0.2 - S /500 or [D+ 1/500] S = 0.2...S_h = e^(-t / 500), and S_p = 100....S_g = c1 e^(- t / 500) + 100...initial conditions yield c1 = - 50....note the long term solution has S = 100, ie. brine of 0.1 kg/L