9.
\includegraphics[max width=\textwidth, alt={}, center]{ab2949b2-11f2-4682-ab0c-25ecee2d665a-4_268_648_1169_623}
Two tanks, \(A\) and \(B\), each have a capacity of 800 litres.
At time \(t = 0\) both tanks are full of pure water.
When \(t > 0\), water flows in the following ways:
- Water with a salt concentration of \(\mu\) grams per litre flows into tank \(A\) at a constant rate
- Water flows from tank \(A\) to tank \(B\) at a rate of 16 litres per minute
- Water flows from tank \(B\) to tank \(A\) at a rate of \(r\) litres per minute
- Water flows out of tank \(B\) through a waste pipe
- The amount of water in each tank remains at 800 litres.
This system is represented by the coupled differential equations
$$\begin{aligned}
& \frac { \mathrm { d } x } { \mathrm {~d} t } = 36 - 0.02 x + 0.005 y
& \frac { \mathrm {~d} y } { \mathrm {~d} t } = 0.02 x - 0.02 y
\end{aligned}$$
Solve the coupled differential equations to find both \(x\) and \(y\) in terms of \(t\).