Two pipes can fill a tank in 30 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 ?  Choices
A) 5 Hours	B) 7 Hours	C) 8 Hours	D) 9 Hours	E) 15 Hours
In 1 hour it fill 1/30 + 1/12 - 1/20 = 1/15   So fill tank fill up in 15 Hours   ANSW15 HoursR:15 Hours
<result>E</result>
Two pipes can fill a tank in 30 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 ?