A cistern is filled by a tap in 8 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?  Choices:  A) 133  B) 143  C) 153  D) 163  E) 173
filling rate - leak rate = net rate   1/8.5 - leak rate = 1/9   leak rate = 2/17 - 1/9 = 1/153   The answer is 153.
<result>C</result>
A cistern is filled by a tap in 8 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?