A pipe can fill a cistern in 12 minutes whereas the cistern when fill can be emptied by a leak in 18 minutes. When both pipes are opened, find when the cistern will be full? Choose the correct answer:
A) 17
B) 16
C) 70
D) 36
E) 12
1/12 - 1/18 = 1/36   36 minutes   Answer: 36
<result>D</result>
A pipe can fill a cistern in 12 minutes whereas the cistern when fill can be emptied by a leak in 18 minutes. When both pipes are opened, find when the cistern will be full?