<PRE><B>Question 150822</B>
A tank can be filled by two pipes separately in 10 and 15 minutes respectively. When a third pipe is used simultaneously with the first two pipes, the tank can be filled in 4 minutes. How long would it take the third pipe alone to fill the tank?


find the rate it would take the first two pipes together.


1/10+1/15=x
25/150=x


25/150y=1
y=150/25
y=6


6 minutes for Pipe A and B together.


1/6+z=1/4


Subtract 1/6 from both sides
z=1/12


So the third pipe can fill the tank in 12 minutes.</PRE>