This first part of this puzzle is quite well known, but is worth another look, as some people might not have seen it before. The second part is a little twist that makes the puzzle slightly more difficult, and it is probably much less familiar to the majority of people.
First, you have to measure exactly four pints of water in a jug. The complication is that you have only two jugs; one that holds three pints and another that holds five pints. Neither jug has any markings, and both are irregular enough in shape that you can't estimate what proportion of the jug is full. That is, you can't work out what four-fifths of the five-pint jug looks like.
You can fill either jug from a tap as many times as you want.
So, how do you get four pints of water in the larger jug?
Here is one way:
- Fill the five-pint jug from the tap,
- Fill the three-pint jug from the five-pint jug (two pints remaining)
- Empty the three-pint jug
- Pour the two pints from the five-pint jug into the three-pint jug
- Fill the five-pint jug from the tap
- Pour one pint from the five-pint jug into the three-pint jug (ie fill up the three-pint jug from two pints to three pints)
- There are four pints in the five-pint jug
And now to the twist. In this part of the puzzle the five-pint jug is too large to go under the tap, so you can only put water in the five-pint jug by pouring it from the three-pint jug.
Again, how do you get four pints of water in the big jug?
Here is one way:
- Fill the three-pint jug from the tap
- Pour the three pints into the five-pint jug
- Fill the three-pint jug from the tap
- Pour two pints from the three-pint jug into the five-pint jug (ie fill up the five-pint jug from three pints to five pints, with one pint left in the three-pint jug)
- Empty the five-pint jug
- Pour the remaining one pint from the three-pint jug to the five-pint jug
- Fill the three-pint jug from the tap
- Pour the three pints from the three-pint jug to the five-pint jug
- There are four pints in the five-pint jug