Three Plus Five Make Four
You have two containers, one that holds 3 liters and one that holds 5 liters, and access to a water source. Place exactly 4 liters of water into a third container (one that holds at least 5 liters, so that you can't simply fill the container to the brim and be done with it). This puzzle dates back at least as far as the 1920s, and quite possibly earlier still. The solution is as follows:
- First, fill the 3-liter container to the brim.
- Pour the 3 liters of water into the 5-liter container.
- Fill the 3-liter container again.
- Pour water from the 3-liter container into the 5-liter container until it is filled to the brim. This should leave you with 1 liter of water in the 3-liter container.
- Empty out the 5-liter container, then pour the 1 liter of water into it from the 3-liter container.
- Fill the 3-liter container once more, then pour the water from both containers into the third container.
- Fill the 5-liter container.
- Fill the 3-liter container from the 5-liter container, leaving 2 liters.
- Empty the 3-liter container. Then, pour the remaining 2 liters from the 5- to the 3-liter container.
- Refill the 5-liter, then fill the 3-liter from the 5-liter container. Leaving 4 liters in the 5-liter container.
- Fill the 5-liter container, then place one part of the bottom down, and tilt it until the water level makes a straight line from the upper edge of the bottom to the bottom edge of the lip.
- Do the same thing with the 3-liter container, and pour it into the 5-liter container.
- 2.5 + 1.5 = 4
ExamplesAlternate Reality Game
- Perplex City used this one on a card.
- In the movie Die Hard with a Vengeance, the two main characters must solve this type of puzzle in order to disarm a bomb near a public fountain.
In the original script, the bomb was a Chekhov's Gun, as the movie would end with McClane planting the bomb on the bad guy's helicopter, and upon discovery one of them asks if anyone has a four gallon jug (in the final script the bomb was used for something else entirely without the puzzle being brought up again).
- This puzzle appears in Knights of the Old Republic on Manaan, and solving it is a prerequisite for the "good" ending on that planet.
- This puzzle and several variants appear in the Professor Layton series.
- A slight variant with different values in Zork Zero, a game almost entirely composed of stock puzzles.
- The Castle of Dr. Brain has a form of this with its hourglass puzzle. Measure 40 seconds with a 25 and 15-second hourglass.
- A variation of this puzzle occurs in Fatal Hearts, where you need ⅔ cup of flour but have only an 800ml cup and a 100ml cup. The puzzle's not too hard if you're good at conversions.
- An apparently more complex version of this classic puzzle appears at the end of Sherlock Holmes Versus Arsène Lupin, where you have to measure exactly 478cc of black powder with only 250cc, 80cc and 12cc containers at your disposal. I say apparently only, because 1×250 + 19×12=478, so you don't actually have to use the pour-bigger-into-smaller subtraction trick at all to solve it.
- It can be done much faster with the pour-bigger-into-smaller trick, though: 1x250 + 3x80 − 1x12=478.
- The original version also appears in "Sherlock Holmes And The Mysteryof The Mummy" as well as "Sherlock Holmes And The Mysteryofthe Persian Carpet" (Which borrowed every other puzzle too)
- Appears in RuneScape, among other Stock Puzzles.
- Appears in Phantasmat as an unusual method of switching on a generator.
- Done with lava in the last chapter of the Wii Carmen Sandiego game.
- In Sir Basil Pike Public School, if you pick the boy's path, you must solve this puzzle for the janitor before heading to the tennis court. (The girl's path has a Knights and Knaves puzzle instead.)
- The final level of Tomb Raider: The Last Revelation contains this exact puzzle and two variants.