Pipe A can fill a tank in 4 hours. Due to a leak at the bottom, it takes 6 hours for the pipe A to fill the tank. In what time can the leak alone empty the full tank?  Choices:
A) 12
B) 67
C) 95
D) 36
E) 66

Rationale: "let the leak can empty the full tank in x hours 1 / 4 - 1 / x = 1 / 6 = > 1 / x = 1 / 4 - 1 / 6 = ( 3 - 2 ) / 12 = 1 / 12 = > x = 12 . answer : a"
Answer: A