Two pipes can fill a tank in 10 minutes and 15 minutes. An outlet pipe can empty the tank in 30 minutes. If all the pipes are opened when the tank is empty, then how many minutes will it take to fill the tank?
Choose one:
A) 7.0
B) 7.5
C) 8.0
D) 8.5
E) 9.0
Let V be the volume of the tank.   The rate per minute at which the tank is filled is:   V/10 + V/15 - V/30 = 2V/15 per minute   The tank will be filled in   15/2= 
<gadget id="calculator">  15/2</gadget>
<output>15/2 = around 7.5</output>
7.5 minutes.   The answer is 7.5.
<result>B</result>
Two pipes can fill a tank in 10 minutes and 15 minutes. An outlet pipe can empty the tank in 30 minutes. If all the pipes are opened when the tank is empty, then how many minutes will it take to fill the tank?