Two pipes can fill a tank in 10 and 12 hours respectively while a third pipe can empty the full tank in 20 hours. If all three pipes operate simultaneously, in how much time will the tank be filled?
Answer: B
Net rate = \(1/10 + 1/12 - 1/20\). LCM of 10, 12, 20 is 60. Net rate = \((6+5-3)/60 = 8/60 = 2/15\) tank/hour. Time to fill the tank = 15/2 hours = 7.5 hours, which is 7 hours and 30 minutes.