Question 230679
a water tank has an inlet pipe and a drain pipe.
 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?
:
Here's another way to do it
;
Let t = time to empty with both valves opened
:
Let an empty tank = 1
therefore:
+ for emptying; - for filling
:
{{{t/30}}} - {{{t/45}}} = 1
Multiply by 90, results:
3t - 2t = 90
t = 90 min to drain the tank with both valves open