Pipe A and Pipe B can fill a tank in 10 hours and 20 hours respectively. Pipe C can empty the tank in twice the time taken by Pipe B to fill the tank. Find the time taken by Pipe C to empty the water filled by Pipe A and Pipe B together in 5 hours.

ATQ, Pipe A can fill the tank in 10 hours. Pipe B can fill the tank in 20 hours. Pipe C can empty the tank in twice the time taken by Pipe B. So, time taken by Pipe C to empty the full tank = 20 × 2 = 40 hours. Now take total tank capacity as 40 units, because 40 is the LCM of 10, 20 and 40. Pipe A’s 1-hour work = 40 ÷ 10 = 4 units. Pipe B’s 1-hour work = 40 ÷ 20 = 2 units. Pipe C’s 1-hour emptying work = 40 ÷ 40 = 1 unit. Pipe A and Pipe B together fill in 1 hour = 4 + 2 = 6 units. In 5 hours, Pipe A and Pipe B together will fill: 6 × 5 = 30 units. Now Pipe C empties 1 unit in 1 hour. So, Pipe C will empty 30 units in: 30 ÷ 1 = 30 hours.