coffee machine

[AnalogKid2DigitalMan]

For my study I have made a coffee machine where I can select coffee, hot water, cold water.

Also you can select the strength and add sugar and milk.

Now I am trying to add a watertank that reacts on the selection of coffee or water.

So when you select a cup of coffee the level in the watertank has to get less and after for example 20 cups of coffee you have to push a button to fill the tank.

Can somebody help me with this question and maybe send me a example?

Thanks in advanced