A pipe can fill a cistern in 18 minutes whereas the cistern when fill can be emptied by a leak in 20 minutes. When both pipes are opened, find when the cistern will be full? Choices.
A) 17
B) 180
C) 70
D) 120
E) 12
1/18 - 1/20 = 1/180   180 minutes   Answer: 180
<result>B</result>
A pipe can fill a cistern in 18 minutes whereas the cistern when fill can be emptied by a leak in 20 minutes. When both pipes are opened, find when the cistern will be full?