Follow a mixing tank by computer

You will model a tank whose volume grows, and follow the salt in it with Euler's method. The tank holds $300\ \text{L}$ and $30\ \text{kg}$ of salt at the start. Brine of $0.2\ \text{kg/L}$ enters at $4\ \text{L/min}$, and the mixture leaves at $2\ \text{L/min}$.

The idea. The volume is $V = 300 + 2t$. The salt satisfies $\dfrac{dQ}{dt} = 4 \times 0.2 - 2\dfrac{Q}{V}$. The exact solution is $Q = \dfrac{240t + 0.8t^2 + 9000}{300 + 2t}$. Euler's method with a small step should agree with it.

Your tasks.

1. Finish volume(t). Return the volume in litres at time t minutes.
2. Finish salt_rate(Q, t). Return $\dfrac{dQ}{dt}$ in kilograms per minute.
3. Finish simulate(t_end, h). Start with Q = Q0 and repeat the step round(t_end / h) times. Return the salt at the end.