Mixtures for brewing coffee (basic mixture model is da/dt= (rate in) - (rate out) )

I have an interesting question here... For my Diff EQ class, we are doing projects, making mathematical models of different things.

I am doing a model of brewing coffee, looking specifically at the mixing aspect. I was trying to start by making a basic model, than incorporate temperature, evaporation, etc. Basic mixtures model is:

da/dt= (rate in) - (rate out)

This would be the amount of salt (or in this case coffee) remaining per amount of time. I want to find the amount of coffee that comes out when mixed for a set amount of water and coffee. How would I set an equation for this up?