A pipe can fill a cistern in 20 minutes whereas the cistern when full 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 from the following answers:
A) 17 B) 16 C) 70 D) 13 E) 12
1/20 - 1/28 = 1/70   70 minutes   Answer: 70
<result>C</result>
A pipe can fill a cistern in 20 minutes whereas the cistern when full can be emptied by a leak in 28 minutes. When both pipes are opened, find when the cistern will be full?