Two pipes A and B can separately empty a cistern in 45 min and 30 min respectively. There is a third pipe in the top of the cistern to fill it. If all the three pipes are simultaneously opened, then the cistern is full in 90 min. In how much time, the third pipe alone can fill the cistern?	Choose the correct answer.
A) 22.5 min  B) 15 min  C) 30 min  D) 20 min  E) 25 min
Work done by the third pipe in 1 min = (1/45 + 1/30) - 1/60 = 4/90.   [+ve sign means filling]   The third pipe alone can fill the cistern in  90/4= 
<gadget id="calculator"> 90/4</gadget>
<output>45/2 = around 22.5</output>
22.5 min.   22.5 minNSWER:22.5 min
<result>A</result>
Two pipes A and B can separately empty a cistern in 45 min and 30 min respectively. There is a third pipe in the top of the cistern to fill it. If all the three pipes are simultaneously opened, then the cistern is full in 90 min. In how much time, the third pipe alone can fill the cistern?