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Die Hard Water Puzzle

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Description

The Water Puzzle from the Die Hard 3 film

Resources

two water containers marked 5 litre and 3 litre
(I use old sweet jars [free from local shop] and down size to pints)
Or, one could use two empty plastic jugs, cut down to the correct capacity
(I used two different sized washer fluid jugs from the recycling bin)

Instructions

You might remember this puzzle from Die Hard with a Vengeance where Bruce Willis and Samuel L. Jackson have to diffuse a bomb by placing a 4 gallon jug of water on a set of scales...

Gallons are big, so we used litres, or you can use even smaller measures, if indoors and looking to reduce mess, but 3 & 5 are the point.
Take your 3 & 5 measure, and make 4.

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.

Solutions Below:
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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.

There are two ways (plus possibly one other!) to solve this, maybe the question could be modified to say the 5 litre can doesn't fit under the tap...

Number 1

Fill the 5 litre can from the tap
Empty the 5 litre can into the 3 litre can - leaving 2 litres in the 5 litre can.
Pour away the contents of the 3 litre can.
Fill the 3 litre can with the 2 litres from the 5 litre can - leaving 2 litres in the 3 litre can.
Fill the 5 litre can from the tap.
Fill the remaining 1 litre space in the 3 litre can from the 5 litre can.
Leaving 4 litres in the 5 litre can.

Number 2

Fill the 3 litre can from the tap.
Empty the contents of the 3 litre can into the 5 litre can.
Fill the 3 litre can from the tap.
Empty the contents of the 3 litre can into the 5 litre can. - Leaving the 5 litre can full and 1 litre in the 3 litre can.
Pour away the contents of the 5 litre can
Pour the 1 litre from the 3 litre can into the 5 litre can.
Fill the 3 litre can from the tap.
Empty the contents of the 3 litre can into the 5 litre can.
Leaving 4 litres in the 5 litre can.

Lastly... number 3 on of our Scout patrols came up with, thinking once again outside the box, as good Scouts do. ;-)

Because their 3L jug was open top and the same height as the 5L one, just a larger diameter, the 3L fit fully inside the 5L.
Our Scouts did this...
Fill the 5L jug to the top.
Use the 3L jug to press into the 5L jug displacing (overflowing) 3L of water, leaving 2L in the jug.
Pour that 2L into the 3L.
Refill the 5L.
Repeat the displacement trick, leaving 2L again.
Pour the 2L from the 3L into the 5L, and you again have 4L

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