A cistern is normally filled in 4 hours but takes two hours longer to fill because of a leak in its bottom. If the cistern is full, the leak will empty it in?  Choose the correct choice
A) 33
B) 88
C) 40
D) 99
E) 12

Rationale: "1 / 4 - 1 / x = 1 / 6 x = 12 answer : e"
Answer: E