A pipe can fill a cistern in 6 hours. Due to a leak in the bottom it is filled in 7 hours. When the cistern is full, in how much time will it be emptied by the leak?

Explanation

Given
A pipe can fill a cistern in `6` hours
Due to a leak in the bottom it is filled in `7` hours
Part of cistern filled in `1` hours = `1/6`
Due to leak part of cistern filled in `1` hours = `1/7`
Part of Cistern empited in '1' hours by leak alone `= 1/6 - 1/7`
` (7-6)/ 42`
`1/42`
So, cistern can be empited by leak in `42` hours.