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?
Choices
A) 17 minutes
B) 87 minutes
C) 70 minutes
D) 27 minutes
E) 11 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?