Pipe A and Pipe B can fill a tank in 7 hours and 14 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 3 hours.

ATQ, Pipe A can fill the tank in 7 hours. Pipe B can fill the tank in 14 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 = 14 × 2 = 28 hours. Now take total tank capacity as 28 units, because 28 is the LCM of 7, 14 and 28. Pipe A’s 1-hour work = 28 ÷ 7 = 4 units. Pipe B’s 1-hour work = 28 ÷ 14 = 2 units. Pipe C’s 1-hour emptying work = 28 ÷ 28 = 1 unit. Pipe A and Pipe B together fill in 1 hour = 4 + 2 = 6 units. In 3 hours, Pipe A and Pipe B together will fill: 6 × 3 = 18 units. Now Pipe C empties 1 unit in 1 hour. So, Pipe C will empty 18 units in: 18 ÷ 1 = 18 hours.