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## Main Question or Discussion Point

I am having major problems understanding these types of questions, where you will have water and in will be chemicals mixing at a certain rate, and coming out of another tube at a rate, and then the question is to find out certain things, concentration at a time or whatever.

My question, the equation takes the form

dx/dt = in - out

x is the amount of chemical

The in is the rate coming in, however it seems the out part is usually the number multiplied by some other stuff. The question is, how do I know what to put in ''the other stuff''?

My question, the equation takes the form

dx/dt = in - out

x is the amount of chemical

The in is the rate coming in, however it seems the out part is usually the number multiplied by some other stuff. The question is, how do I know what to put in ''the other stuff''?