There is a glycol tank which, to begin with, contains a mixture of glycol and water in roughly equal amounts (give or take 10%). In addition to being very imprecise about the glycol percentage, I also don’t know for certain how much liquid in total is in the tank, but it is several hundred litres.

The contents of the tank are leaking out at a rate of 2 millilitres a second. At the same time pure water is leaking into the tank at a rate of 1 millilitre per second, and mixing instantaneously with the tank contents.

At any point in time there is, say, ‘g’ millilitres of glycol and ‘w’ millilitres of water in the tank. At the point when ‘w minus g’ is at a maximum, what is the glycol concentration?