<PRE><B>Question 258175</B>
Given, a full tank can be emptied in 30 minutes if the drain is opened.
So in 1 minute 1/30 of the tank can be emptied.   
And an empty tank can be filled in 45 minutes with the inlet pipe opened.
So in 1 minute 1/45 of the tank can be filled.
If both are opened, in one minute, [(1/30)-(1/45)]of the tank can be emptied.
Time taken to empty the full tank = 1/[(1/30)-(1/45)]
                                  = 1/[(45-30)/45*30]
                                  = 45*30/15
                                  = 3*30
                                  = 90 minutes    </PRE>