With only the small valve open, all of the liquid can be drained from a large vat in 4 hours. With only the large valve open, all of the liquid can be drained in 2 hours. How long would it take to drain the vat with both valves open?
Let x be the amount of liquid in the vat.
The small valve drains all of the liquid in 4 hours; therefor the rate of flow from the small valve is x/4 per ...
A 'Separately and Both Together' problem is solved, and the general formula of this problem type is discussed.