There's a well-known puzzle about a bath with hot and cold taps that work at different rates, and a plughole that empties the bath. Well, we have for you a similar puzzle about a water tank.
Using a bucket and carrying water from a nearby pond, Fred can fill the water tank in ten minutes.
Using the same bucket and water source John can fill the water tank in four minutes.
However, John works twice as fast as Fred works! This would seem incorrect, as you would think that the times should be five minutes and ten minutes or four minutes and eight minutes.
The reason for this apparent discrepancy is the hole in the water tank. Fred has to overcome more leakage than John, hence his time is more than twice as long.
The question is not how long the men take to fill the water tank, the question is how long does it take for a full water tank to leak away and become an empty water tank.
Without the leak, Fred should fill the water tank in eight minutes (twice John's four minutes), but he takes ten minutes - an extra two minutes.
As Fred takes six minutes longer than John to fill the water tank he has to overcome six minutes of extra leakage.
So his two minutes of work overcomes six minutes of leakage, which means he is working three time as fast as the leak.
It takes him ten minutes to fill the water tank, so it will be empty 30 minutes after he starts, which is 20 minutes after he has filled it.
The tank empties in 20 minutes, whoever filled it.