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 \(t = 0\),
- find \(S\) in terms of \(t\),
- 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)