<PRE><B>Question 304582</B>
A water tank has an inlet pipe and a drainpipe. 
A full tank can be emptied in 30 minutes if the drain is opened and an empty
 tank can be filled in 45 minutes with the inlet pipe opened.
 If both pipes are accidentally opened when the tank is full, then how long
 will it take to empty the tank?
:
Let t = time required to empty the tank with both valves open
:
Let the completed job = 1 (an empty tank)
:
Draining is positive, filling is negative
{{{t/30}}} - {{{t/45}}} = 1
multiply equation by by 90, results:
3t - 2t = 90
t = 90 minutes with both valves open
;
:
Check solution
90/30 - 90/45 = 
3 - 2 = 1</PRE>