Hint to Riddle #22: The 3 & 5 Litre Die Hard Water Puzzle

22. You have a 3 and a 5 litre water container, each container has no markings except for that which gives you it's total volume. You also have a running tap. You must use the containers and the tap in such away as to exactly measure out 4 litres of water. How is this done?
Can you generalise the form of your answer?

water jugs to illustrate die hard water puzzle This puzzle gets a surge of hits a few times a year and I know that somewhere Die Hard: With a Vengeance has been on TV.

The puzzle is pure in the sense that it doesn't rely on guessing what a half-full can looks like or balancing anything. It can be done exactly by simply filling and pouring away.

This puzzle has two completely different solutions both equally valid. In order to get 4 litres in the 5 litre can we are either...

...going to have to get one litre in to the 5 litre can so we can top it up with a full 3 litres. And know we have 4 litres.

OR

...get 2 litres in to the 3 litre can so we can fill the 5 litre can from the tap and pour away exactly 1 litre into the 3 litre can.

If you like that is the step before the end.


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