
Changing Atmosphere
I came up with my own problem. (Emo)
There is nothing complicated.
Suppose a room (air tight) is filled with $\displaystyle 100\%$ oxygen.
In the room there is a machine which takes in $\displaystyle 5$ liters/minute of air and gives up pure nitrogen.
Find a way to model the amount of oxygen at any given time.
(Of course use obvious assumption necessary).

Nice problem indeed, thank you. I guess we assume that nitrogen and oxygen mix immediately in the whole volume.
The whole volume is $\displaystyle V$, the proportion of oxygen at time $\displaystyle t$ (in minutes) is $\displaystyle x(t)$, so $\displaystyle x(0)=1$ and probably $\displaystyle x(t)\to_{t\to\infty} 0$.
During an infinitesimal time interval $\displaystyle \Delta t$, the initial volume of oxygen $\displaystyle Vx(t)$ becomes $\displaystyle Vx(t+\Delta t)=Vx(t)5\Delta t x(t)$ (since $\displaystyle 5\Delta t$ is the volume that was taken away, among which a proportion $\displaystyle x(t)$ was composed of oxygen). So:
$\displaystyle \frac{x(t+\Delta t)x(t)}{\Delta t}=\frac{5}{V}x(t)$.
Stated differently, $\displaystyle \frac{dx}{dt}=\frac{5}{V}x(t)$, so that $\displaystyle x(t)=e^{\frac{5}{V}t}$. The proportion hence decreases exponentially fast.
Laurent.

Here is how I would do it.
Let $\displaystyle y(t)$ be a function which represents amount of oxygen at time $\displaystyle t$ in minutes.
Notice that $\displaystyle y'(t)$ is the rate of change.
Thus, $\displaystyle y'(t) = \text{ rate out }  \text{ rate in }$.
The rate in is amount of oxygen coming in per minute is $\displaystyle 5\cdot \tfrac{y}{V}$.
The rate out is $\displaystyle 0$.
Thus, $\displaystyle y'(t) =  \tfrac{5}{V}y$ with $\displaystyle y(0) = V$.