P can fill a tank in 5 hours. Q can fill the same tank in 10 hours. R can empty the same tank in 20 hours. How much time will all the three take together to fill the same tank?

4 hours

total efficiency = 4 + 2 - 1 = 5

Time taken taken by P, Q, R to fill the tank = $\frac{total\;capacity}{efficiency}$

                               = $\frac{20}{5}$ = 4hrs