A water tank is $$\frac{2}{5}$$ th full. Inlet A can fill the tank in 12 minutes while an outlet pipe B can empty it in 6 minutes. If both the pipes are open, how long will it take to empty or fill the tank completely?
Solution
Pipe A can fill in 12 hours.
Pipe B can empty in 6 hours.
Let's assume total work is 60.
Efficiency of A = 5
Efficiency of B = -10
Tank is $$\frac{2}{5}$$ th full. It means that it's 24 parts filled.
If both pipes A and B are opened. Then efficiency will be -5.
So $$\frac{\text{24 parts}}{-5}$$
-4.8 min. (Here negative sign represent for emptying.)
