First order modelling ODE - pollution in lake

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

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

How would I form a differential equation for this scenario?