4. During an industrial process, the mass of salt, \(S \mathrm {~kg}\), dissolved in a liquid \(t\) minutes after the process begins is modelled by the differential equation $$\frac { \mathrm { d } S } { \mathrm {~d} t } + \frac { 2 S } { 120 - t } = \frac { 1 } { 4 } , \quad 0 \leq t < 120$$ Given that \(S = 6\) when \(t = 0\),

1. find \(S\) in terms of \(t\),
2. calculate the maximum mass of salt that the model predicts will be dissolved in the liquid at any one time during the process.
   (4)(Total 12 marks)