A pipe can fill a cistern in 20 minutes whereas the cistern when fill can be emptied by a leak in 60 minutes. When both pipes are opened, find when the cistern will be full?  Choose the correct option: A) 17 minutes B) 87 minutes C) 30 minutes D) 27 minutes E) 11 minutes
1/20 - 1/60 = 1/30   30 minutes   Answer:30 minutes
<result>C</result>
A pipe can fill a cistern in 20 minutes whereas the cistern when fill can be emptied by a leak in 60 minutes. When both pipes are opened, find when the cistern will be full?