Two pipes can fill the cistern in 10hr and 12 hr respectively, while the third empty it in 20hr.
If all pipes are opened simultaneously, then the cistern will be filled in	Choose the correct choice:  A) 10.5  B) 8  C) 6  D) 7.5  E) 8.5
First pipe fills cistern in 10 hours.   So, in 1 hour, pipe fills 1/10 of the cistern capacity   2nd pipe fills cistern in 12 hours.   So, in 1 hour, 2nd pipe fills 1/12 of the cistern capacity   3rd pipe empties cistern in 20 hours   So, in 1 hour, 3rd pipe empties 1/20 of the cistern capacity   Net volume added to cistern per hour = Volume filled by 1st pipe per hour + Volume filled by 2nd pipe per hour - Volume emptied by 3rd pipe per hour   = 1/10 + 1/12 - 1/20   =(6 + 5 - 3) / 60 = 8/60   =&gt; Net volume added to cistern per hour = 2/15   So, Cistern will be fill in  15/2= 
<gadget id="calculator"> 15/2</gadget>
<output>15/2 = around 7.5</output>
7.5 hours   So, answer is 7.5
<result>D</result>
Two pipes can fill the cistern in 10hr and 12 hr respectively, while the third empty it in 20hr.
If all pipes are opened simultaneously, then the cistern will be filled in