Two pipes can fill a tank in 12 hours and 18 hours, respectively, while a third pipe can empty it in 8 hours. How long (in hours) will it take to fill the empty tank if all three pipes are opened simultaneously?
Show answer & explanation
Correct answer: D
Explanation
The correct answer is option 4 i.e. 72.
Total capacity = 72 (LCM of 12, 18, and 8)
Filling rate of two pipes = (72/12) + (72/18) = 6 + 4 = 10 units/hour
Empty rate of third pipe = 72/8 = 9 units/hour
Total filling rate of three pipes = 10 - 9 = 1 unit/hour
Time required for three pipes = 72/1 = 72 hours
Hence, it will take three pipes 72 hours to fill the tank.