Mixture Problems

In algebra, there are word problems where you have to figure out how much material you have to add to a mixture to get a certain concentration or amount. In calculus, naturally, the problem has to be harder: for example, the mixture is changing; material is going into the mixture, and material is going out. You have to find out how much mixture or how much of the solvent is present after a specific amount of time. Let's look at the following exercise to better understand this concept.

Exercise 12.12: Solving Mixture Problems – Part 1

A tank contains 82 gallons of brine in which 18 pounds of salt is dissolved. Brine containing 3 pounds of dissolved salt per gallon flows into the tank at the rate of 5 gallons per minute. The mixture, which is kept uniform by stirring, flows out of the tank at a rate of 2 gallons per minute. How much salt is in the tank at the end of 39 minutes?

As you can imagine, this kind of problem leads to some complicated differential...