Two pipes can fill a tank in 18 and 12 hours, while third pipe will make the tank empty in 20 hours. If all three pipes operate simultaneously. In how many hours the tank will be filled ?	Choose one.
A) 5.25 Hours B) 7.25 Hours C) 8.25 Hours D) 9.25 Hours E) 11.25 Hours
In 1 hour it fill 1/18 + 1/12 - 1/20 = 4/45   So fill tank fill up in 45/4= 
<gadget id="calculator">45/4</gadget>
<output>45/4 = around 11.25</output>
11.25 Hours   ANSW11.25 HoursR:11.25 Hours
<result>E</result>
Two pipes can fill a tank in 18 and 12 hours, while third pipe will make the tank empty in 20 hours. If all three pipes operate simultaneously. In how many hours the tank will be filled ?