Question

A pipe can fill a cistern in 6 hours due to a leak in its bottom it is filled in 7 hours

Answer 1

what do you want to tell

Answer 2

If you ignore that the leaking rate is dependent of fluid height, which is what you have to do in order to get an estimate from this limited data, you can think like this: in 7h you pumped 1.166 x total volume, so in 7h it was leaking 16.6%. It will go to 100% in 42.2h.