You must defuse a bomb by placing exactly 4 liters of water on a scale. The only problem is that you only have one 5 liter and one 3 liter tank available! This classic riddle, made famous by the movie Die Hard, may seem impossible without a measured glass, but it is, in fact, quite simple.
If you are only interested in the solution, click here to skip to the answer. If, on the other hand, you want to solve the riddle by yourself with some suggestions, read on.
Steps
Method 1 of 2: Getting to the Solution (Tips)
Step 1. Simplify the question and your possibilities
Leaving the film aside for a moment, think about the riddle in the simplest terms possible. What information is available to you, what is your goal and what are your options? Here is a way to rephrase the riddle in a simpler way:
You have two empty water tanks in your hand. One can hold 3 liters and the other 5. You must use these two tanks to measure exactly 4 liters of water. You have an infinite amount of water available to do this.
Step 2. Ask yourself where you will keep the 4 liters of water
In order to measure 4 liters, you will have to contain it somewhere. As John McClane correctly deduces, it is not possible to get 4 liters of water into a 3-liter tank, so the correct solution requires pouring the necessary amount of water into the 5-liter tank.
Step 3. Remember that, in all the best riddles, you have everything you need to find the solution
It is not necessary to use another tank, measure the water level by eye or find fractions of the tanks. You have two tanks and an infinite amount of water. Can you use these elements to get 4 liters? More specifically, how can you use 3 and 5 to get 4?
- An unlimited amount of water means you can use or pour as much water as you like.
- You won't be able to tell how much water is in a tank if you don't fill it completely.
Step 4. Understand that this is basically a simple math problem
Ignore the water and canisters for a moment if you are still stuck. How is it possible to get 4 by adding and subtracting 5 and 3? This is the only question you need to answer, regardless of whether the numbers represent liters. Adding or pouring water just means adding and subtracting.
Method 2 of 2: Find the Solution (Correct Answers)
Solution 1
Step 1. Fill the 5 liter tank completely
You will necessarily have to fill it to the brim or you will not know how many liters it will contain.
Step 2. Use the water in the 5 liter tank to fill the 3 liter tank
At this point, you will have got a full 3 liter canister and a 5 liter canister with 2 liters inside.
Step 3. Pour the water contained in the 3 liter tank
At this point, you will have obtained an empty 3 liter canister and a 5 liter canister with 2 liters inside.
Step 4. Transfer the water from the 5 liter to the 3 can
There are now 2 liters of water in the 3-liter tank and the 5-liter tank is empty.
Step 5. Fill the 5 liter can to the brim
You will now have 2 liters in the 3 liter tank and 5 in the 5 tank. This means that there is room for one liter in the 3 liter tank.
Step 6. Use the water in the 5 liter tank to fill the 3 liter tank
Fill the last liter of space in the 3-liter tank with the water contained in the 5-liter tank. At this point you will have a full 3-liter tank and 4 liters in the 5.
Solution 2
Step 1. Fill the 3 liter tank completely
Step 2. Pour the water into the 5 liter tank
You will now have an empty 3 tank and a 5 liter tank with 3 liters.
Step 3. Fill the 3 liter tank with water again
You will now have a full 3 canister and a 5 liter canister.
Step 4. Fill the 5 liter tank with the water contained in the 3 tank
You will now have one liter in the 3 tank and 5 in the 5 tank. This is because, after the last step, there was only 2 liters of space left in the large tank.
Step 5. Turn the contents of the 5 liter tank upside down and fill it with the liter contained in the 3 liter tank
You will now have an empty 3 liter canister and a 5 liter canister with 1 liter.
Step 6. Fill the 3 liter tank
You will now have a 3 liter tank full of water and a 5 liter tank with 1 liter.
Step 7. Move the 3 liters of water to the 5 tank to get 4 liters
Just pour the contents of the full 3-liter tank into the 5-liter tank, which only contained one liter; 1 + 3 = 4 and a defused bomb.
