Pipes A and B can fill a tank in 5 and 6 hours respectively. Pipe C can empty it in 12 hours. If all the three pipes are opened together, then the tank will be filled in  Choose one
A) 1×13/17 hours  B) 2×8/11 hours  C) 3×9/17 hours  D) 4×1/2 hours  E) 5×1/2 hours
(a+b+c) can fill in (1/5+1/6-1/12) =3x9/17   ANSWER:3×9/17 hours
<result>C</result>
Pipes A and B can fill a tank in 5 and 6 hours respectively. Pipe C can empty it in 12 hours. If all the three pipes are opened together, then the tank will be filled in