Two pipes can fill a tank in 20 minutes and 12 minutes. An outlet pipe can empty the tank in 10 minutes. If all the pipes are opened when the tank is empty, then how many minutes will it take to fill the tank?  Choose the correct choice from the following options:
A) 24  B) 26  C) 28  D) 30  E) 32
Let V be the volume of the tank.   The rate per minute at which the tank is filled is:   V/20 + V/12 - V/10 = V/30 per minute   The tank will be filled in 30 minutes.   The answer is 30.
<result>D</result>
Two pipes can fill a tank in 20 minutes and 12 minutes. An outlet pipe can empty the tank in 10 minutes. If all the pipes are opened when the tank is empty, then how many minutes will it take to fill the tank?