Pipe A can fill a tank in 6 hours. Due to a leak at the bottom, it takes 9 hours for the pipe A to fill the tank. In what time can the leak alone empty the full tank?  Choose one:	A) 11	B) 15	C) 18	D) 77	E) 12
Let the leak can empty the full tank in x hours 1/6 - 1/x = 1/9   =&gt; 1/x = 1/6 - 1/9 = (3 - 2)/18 = 1/18   =&gt; x = 18.   Answer: 18
<result>C</result>
Pipe A can fill a tank in 6 hours. Due to a leak at the bottom, it takes 9 hours for the pipe A to fill the tank. In what time can the leak alone empty the full tank?