Pipe A can fill a tank in 12 hours. Due to a leak at the bottom, it takes 18 hours for the pipe A to fill the tank. In what time can the leak alone empty the full tank? Choose the correct answer:	A) 36	B) 87	C) 40	D) 37	E) 86

Rationale: "let the leak can empty the full tank in x hours 1 / 12 - 1 / x = 1 / 18 = > 1 / x = 1 / 12 - 1 / 18 = ( 3 - 2 ) / 36 = 1 / 36 = > x = 36 . answer : a"
Answer: A