A cistern is filled by a tap in 3 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 long will it take the leak to empty it?	Choose the correct choice from the following answers:
A) 11
B) 12
C) 29
D) 28
E) 29

Rationale: 2 / 7 - 1 / x = 1 / 4 x = 28 answer : d
Answer: D