Very difficult .. urgent

During an industrial process, the mass of salt, S kg, dissolved in a liquid t minutes after the process begins is modeled by the differential equation

dS/dt + 2S/(120 – t) = 1/4 , 0<= t < 120

Given that S = 6 when t = 0,
(a)find S in terms of t,
(b)calculate the maximum mass of salt that the model predicts will be dissolved in the liquid at any one time during the process.