Two pipes can fill a tank in 10 hours and 12 hours. While the 3rd pipe empties the full tank in 20hours. If all the 3 pipes are open in how much time will the tank be filled?	Choose the correct choice from the following choices.
A) 6hr  B) 5hr  C) 7hr 30min  D) 8hr 20min  E) 9hr
Net part filled in 1 hour = 1/10 + 1/12 - 1/20 = 8/60 = 2/15   The tank will be full in 15/2 hours = 7hr 30min   Answer is 7hr 30min
<result>C</result>
Two pipes can fill a tank in 10 hours and 12 hours. While the 3rd pipe empties the full tank in 20hours. If all the 3 pipes are open in how much time will the tank be filled?