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?	Pick:
A) 16 hours  B) 15 hours  C) 18 hours  D) 17 hours  E) None of these
Explanation:   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 IS 18 hours
<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?