[SOLVED] Application Differential Equation: mixture problem.

An aquarium pool has volume 2 http://www.webassign.net/images/multiply.gif 10^6 liters. The pool initially contains pure fresh water. At *t* = 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 for *S*(*t*), the mass of salt in the pool at time *t*."

Any help is appreciated.