I always have trouble with this modelling questions, on how to approach them.

An initially unpolluted lake of 10^9 litres has a river flowing through it at 1,000,000 litres per day. A factory is built which discharges 10,000 litres per day of pollutant into the lake. Assuming that the total volume of liquid in the lake remains constant at 10^9 litres:

How would I form a differential equation for this scenario?