At a dairy plant, a milk tank can be filled in 4 hours (using the in-valve). Using the out-valve, the tank can be emptied in 8 hours. If both valves are open and milk is being pumped into the tank, how long will it take to fill the tank?
Not sure this is in the right category, but hey ho ...
Let’s assume that the milk flows in at 10 kg per min. As it takes twice as long to empty it is flowing out at 5 kg per min.
If both valves are open, then the net (or total) increase = rate of inflow - rate of outflow, or 5 kg per min, so it would take 8 hrs to fill.