Let the number of hours which leak takes to empty a cistern be t. A cistern which could be filled in 9 hours takes one hour more to be filled owing to a leak in its bottom i.e. with the leak it is filled in 10 hours ⇒ 1/9 – 1/t = 1/10 ⇒ 1/t = 1/90 ⇒ t = 90 hours