In calculus and differential equations, a standard example of word problems are mixing problems, with some number of tanks, and brine often being an output of the system. With one tank, I can imagine some relation to real world scenarios, as people actually make brine, or maintaining aquariums (perhaps not varying salt content, but doing something like controlling pH).
What I have more trouble motivating with good concrete practical applications is systems where multiple tanks are involved and the tanks flow into each other at possibly different rates, though no doubt there are many examples from environmental sciences, chemistry, etc. Can someone provide convincingly practical mixing examples involving multiple tanks (loosely interpreted)?
Best Answer
It can be seen as a simplified model for demographics. You have several geographical areas, each with some input (births and immigration) and some output (death and emigration). Some of the migration goes between the given areas and some go to or come from the outside. Now say you want to know the number of people after a certain time with a given characteristic. With a few assumptions (people with and without that characteristic are equally likely to move / die / have children etc.) you get exactly the same dynamics.