Two pipes can fill a tank in 12 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 the correct option:
A) 5.5 Hours B) 7.5 Hours C) 8.5 Hours D) 9.5 Hours E) 6.5 Hours
In 1 hour it fill 1/12 + 1/12 - 1/20 = 7/60   So fill tank fillup in 60/7 = 8.5 Hours   ANSWER:8.5 Hours
<result>C</result>
Two pipes can fill a tank in 12 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 ?