Here is the question:
I have posted a link there to this thread so the OP can find my work.Physical application of calculus?
A lake has a volume of 10^6 m^3 and an initial pollution of 2 grams per cubic metre. Every day a river flows in 2 x 10^4 m^3 of a lower pollution of 0.2 gram per cubic metre, and the same amount of water flows out to another river. Assume the water is perfectly mixed at any time. How long will it take to reduce the pollution to 1 gram per cubic metre?
ans: 40.5 days