An aquarium pool has volume 2 10^6 liters. The pool initially contains pure fresh water. Att= 0 minutes, water containing 10 grams/liter of salt is poured into the pool at a rate of 60 liters/minute. The salt water instantly mixes with the fresh water, and the excess mixture is drained out of the pool at the same rate (60 liters/minute).

I need to figure out how to "Write a differential equation forS(t), the mass of salt in the pool at timet."

Any help is appreciated.