Pipe A can fill a tank in 6 hours. Due to a leak at the bottom, it takes 12 hours for the pipe A to fill the tank. In what time can the leak alone empty the full tank?  Choose the correct choice.
A) 13 B) 17 C) 18 D) 19 E) 12

Rationale: "let the leak can empty the full tank in x hours 1 / 6 - 1 / x = 1 / 12 = > 1 / x = 1 / 6 - 1 / 12 = ( 2 - 1 ) / 12 = 1 / 12 = > x = 12 . answer : e"
Answer: E