Answer using Linear Models and ODEs:

You are in charge of a paper mill. Your company got a contract to make a few rolls of paper of a particular kind. You need to add a chemical to a tank already containing that chemical in order to enhance the whiteness of the paper. Consider you cannot stop the mill to add the chemical. The tank receives some input at 20 L per minutes and feed the mill at the same rate. If the tank as a volume of 3000 L, that initially there is 0.3 kg of chemical per L in the tank, and that the added compound must be at a fixed concentration, what should be that concentration if you want the output of the tank to become at 0.7 kg per L in exactly 120 minutes of operation?