The Bloke and I watched Die Hard with a Vengeance the other night, which I've never managed to see all the way through, for reasons I'm not really sure of. The only bit that had stuck in my mind was a puzzle that Bruce Willis and Samuel L. Jackson were set by loony villainist Jeremy Irons, in which they had a 3-litre bottle and a 5-litre bottle and had to use them both to put exactly 4 litres of water on a set of scales to prevent a bomb blowing up. They do it so fast in the movie that you can't really follow what happens, and I spent about 25 minutes after that scene blankly watching the movie while the cogs in my mind were turning and trying to figure out how they did it -- and I did! Snaps to me!
So I thought I would post the solution for the next time I watch the movie and can't remember how they did it. Or maybe for anyone who watches the movie, also can't figure it out, and gets on the web to find out how.
Remember they couldn't use an approximate measurement -- i.e. no filling up the 5-litre bottle around four-fifths of the way up.
1. Fill 5L bottle to the top.
2. Fill the 3L bottle using the full 5L bottle, leaving 2 litres in the 5L.
3. Empty the 3L and refill it with the 2 litres you were left with in the 5L.
4. Fill the 5L all the way up again, then pour as much as will fit into the 3L. Since you already have 2 litres in there, only one more litre will fit -- leaving you with 4 litres in the 5L bottle!
Hmm. Now I've written it out it sounds really easy.