A pipe can fill a cistern in 20 minutes whereas the cistern when fill can be emptied by a leak in 28 minutes. When both pipes are opened, find when the cistern will be full?  Choose the correct choice:
A) 65 minutes B) 76 minutes C) 70 minutes D) 62 minutes E) 52 minutes
1/20 - 1/28   = 1/70   70 minutes   Answer:70 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 28 minutes. When both pipes are opened, find when the cistern will be full?