A cistern is filled by a tap in 7 1/2 hours. Due to leak in the bottom of the cistern, it takes half an hour longer to fill the cistern. If the cistern is full how many hours will it take the leak to empty it? Pick one: A) 80 B) 100 C) 120 D) 140 E) 160
filling rate - leak rate = net rate   1/7.5 - leak rate = 1/8   leak rate = 2/15 - 1/8 = 1/120   The answer is 120.
<result>C</result>
A cistern is filled by a tap in 7 1/2 hours. Due to leak in the bottom of the cistern, it takes half an hour longer to fill the cistern. If the cistern is full how many hours will it take the leak to empty it?