1/20 - 1/28 = 1/70
70 minutes
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?
Options:
1: 17 minutes
2: 87 minutes
3: 70 minutes
4: 27 minutes
5: 11 minutes