Two pipes can fill a tank in 15 hours and 4 hours, respectively, while a third pipe can empty it in 12 hours. How long (in hours) will it take to fill the empty tank if all the three pipes are opened simultaneously?

Two pipes can fill a tank in 15 hours and 4 hours, respectively, while

1.	Two pipes can fill a tank in 15 hours and 4 hours, respectively, while a third pipe can empty it in 12 hours. How long (in hours) will it take to fill the empty tank if all the three pipes are opened simultaneously?
A.	30 / 7
B.	20 / 7
C.	50 / 7
D.	15 / 7
Answer» B. 20 / 7