Two pipes can fill a tank in 15 minutes and 12 minutes. The outlet pipe can empty the tank in 20 minutes. If all the pipes are opened when, the tank is empty, then in how many minutes will it take to fill the tank? Pick one.
A) 12 B) 13 C) 11 D) 10 E) 9
Part of tank filled by all three pipes in one minute =   1/15 + 1/12 – 1/20 = (8+10-6) / 120   = 18-6 / 120   = 1/10   So, the tank becomes full in 10 minutes.   ANSWER:10
<result>D</result>
Two pipes can fill a tank in 15 minutes and 12 minutes. The outlet pipe can empty the tank in 20 minutes. If all the pipes are opened when, the tank is empty, then in how many minutes will it take to fill the tank?