A cistern which could be filled in 6 hours takes one hour more to be filled owing to a leak in its bottom. If the cistern is full in what time will the leak empty it?	Pick one:  A) 76 hrs  B) 99 hrs  C) 15 hrs  D) 90 hrs  E) 11 hrs
1/6 - 1/x = 1/10   =&gt; 15 hrs   Answer: 15 hrs
<result>C</result>
A cistern which could be filled in 6 hours takes one hour more to be filled owing to a leak in its bottom. If the cistern is full in what time will the leak empty it?