Putting 4L Into a 5L Jug - Jan 25

Here's a little brain-teaser for you. Here's how it goes:

You have two jugs. One jug has a 3 litre capacity (or gallon if you prefer) and the other has a 5 litre capacity. Question: How do you use only these two jugs to end up with 4 litres in the 5 litre jug without guessing?

Give up? Here's Answer #1:

1. Fill the 3L jug first.
2. Dump contents into 5L jug.
3. Refill the 3L jug again.
4. Dump what you can (2L) into the 5L jug.
5. Empty the 5L jug. You should now have 1L in the 3L jug and an empty 5L jug.
6. Pour the 1L that you have in the 3L jug into the 5L jug.
   
7. Refill the 3L jug for the third time.
8. Empty contents of 3L jug into the 5L jug and there you have it. 1L + 3L = 4L

In an equation it would look like 3+3-5+3=4.

You'll need a minimum of six litres to complete this problem provided you have a large reservoir tank to draw from and dump in. Without that reservoir tank you'll need 9L of water. The problem with that is now you have an unacceptable attrition rate of 5 out of the 9 litres - 56% - being wasted (see step 5). There you go: a math quiz PLUS a conservation message to boot.

Here's Answer #2:

1. Fill the 5L jug
2. Pour 3 of the 5L into the 3L jug
3. Empty the 3L jug
4. Pour the remaining 2L left in the 5L jug into the 3L jug
5. Refill the 5L jug
6. Pour 1L of the 5L jug into the 3L jug and there you have it

The equation becomes 5-3+5 with an excess of a filled 3L jug. In an environmental scenario, only 3L would be wasted without a reservoir but you would need to use a minimum of 10 Litres in order to do it this way.

Johnny Cash