Pipes A and B can fill a tank in 10 hours and 15 hours, respectively, and pipe C can empty the filled tank in 20 hours. If all three pipes opened together, then in how many hours would the tank be filled?

Correct option is E

Let the capacity of the tank (LCM of 10, 15 and 20) = 60 units
Efficiency of pipes A = 60/10 = 6 units/hour
Efficiency of pipes B = 60/15 = 4 units/hour
Efficiency of pipe C = 60/20 = 3 units/hour (outlet pipe)
Required time = $$\frac{60}{6 + 4 - 3} = \frac{60}{7}$$​ hours