Pipe A can fill the tank in 30 minutes and pipe B in 20min and pipe C can empty the tank in 90 minutes. How long it will take to fill the tank if three of the pipes are operating together? Choose the correct choice from the following:
A) 10.55
B) 12.0
C) 13.1
D) 13.85
E) 15.2
Pipe A fills 1/30th of the tank in a minute and pipe B, 1/20th. Pipe C empties in 90min   (1/30)+(1/20)-(1/90)=(1/x)   13/180=1/x =&gt; x=180/13 =&gt; x=13.85 minutes   Answer: 13.85
<result>D</result>
Pipe A can fill the tank in 30 minutes and pipe B in 20min and pipe C can empty the tank in 90 minutes. How long it will take to fill the tank if three of the pipes are operating together?