A saline solution contains $0.05\rm\,kg/liter $ salt. The saline solution flows into another tank with the speed of $3\rm\,liter/min$. What speed (in units $\rm kg/min$) is salt supplied to the tank?
My answer:
$+3\rm\frac{liter}{min} \times 0.05 \frac{kg}{liter}=0.15 \frac{kg}{min},~~v=?$
The salt is added to the tank with the speed of $0.15\rm\,kg/min$ - which is the correct answer.
But is there any way to describe this with a differential equation?