Setting the movie aside for the moment, think of the riddle in its most basic terms. What do you know, what's your goal, and what are your options? A simpler way to look at the riddle might be:
You have two empty water jugs in your hands. One holds 3 gallons of water, the other holds 5 gallons of water. You need to use these two jugs to measure exactly 4 gallons of water. You have an unlimited amount of water to do so.


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, In order to measure out your 4 gallons, you need to put it somewhere. As John McClane rightly deduces, you couldn't possibly fit it in the 3-gallon jug, which means the right answer must involve getting the right amount of water in the 5-gallon jug.
 The solution does not involve bringing in another jug, or eyeballing the water levels perfectly, or finding fractions of jugs. You have two jugs and an unlimited supply of water. How can you use just those things to get to 4 gallons? More specifically, how can you use 3 and 5 to get to 4?


Unlimited water means you can use or pour out as much as you want.
You are not able to guess exactly how much water is in the jug unless you fill it all the way up.
 Ignore the water and the jugs for a moment if you are still stuck. How can you add or subtract 3's and 5's to get to 4? This is really all you are doing; the numbers just happen to represent gallons. Adding water or dumping it out is really just addition and subtraction.
Simplify the question and your options.
Determine where you will keep the 4 gallons of water.
Know that, like all good riddles, you have everything you need for a solution.
Realize that this is, at heart, a simple math problem.
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