How do you get 4liters of water out of a 5 liter and 3 liter container with a limited supply of water?

Question

How do you get 4liters of water out of a 5 liter and 3 liter container with a limited supply of water?

Answer 1

How limited is the water supply?  The normal way to solve this is as follows:

Fill the 3L container.  Dump it into the 5L.

Fill the 3L again.  Carefully dump it into the 5L until the 5L is full.  The 3L container now contains 1L.

Empty the 5L.

Put the 1L from the 3L container into the 5L.

Fill the 3L and dump into the 5L.

The 5L container now contains 4L.

You've used a total of 9L in order to get here.

In case the above isn't clear, here's a table showing the values at each step:

Step          3L        5L

0 (initial)     0          0

1 (fill 3)      3          0

2 (3->5)      0          3

3 (fill 3)      3           3

4 (3->5)     1(3-2)     5

5 (dump 5)  1           0

6 (3->5)      0           1

7 (fill 3)      3           1

8 (3->5)      0           4

Doing this with less than 9L of water would be fairly tricky.  There is a way to do it with 10L of water, in which the 5L is filled, then dumped into the 3L.  The 3L is dumped, and the remaining 2 liters in the 5L is then put into the 3L.  The 5L is filled again, and then poured into the 3L until it is full.  The 3L is dumped, leaving 4L in the 5L bottle.  The advantage of this is that you don't have to dump the 3L container if you don't want to, allowing you to use this water for something else.

Answer 2

First you must empty both the 5 and the 3 litre containers into one large container. Then you fill the 3 litre container and empty it into the 5. Repeat this step again and you`ll be left with a full 5 litre container, and 1 litre remaining in the 3 litre container. Now empty the 5 litre container back into the large container, and pour the 1 litre that`s left in the 3, back into the 5. Now you have a 5 litre container with 1 litre of water in it, and an empty 3 litre container, so simply fill up the 3 and add it to the 1 in the 5. Now you have 4 litres in the 5 litre container, and an empty 3.