Three water cups, two 4l water

There are three cups with no scales, namely 8l, 5l, and 3l. How to use other tools to make 8l and 5l cups contain 4l water.

 

It is relatively simple to add 8l to 4l, so we can use 5l and 3l to create two 2L products. Then the 8l cup cannot be moved. The key to using 5l and 3l to make 4l is to make a 1l. 1 = 5-4, but this is not feasible, to be manufactured, It is 4, 1 = 3-2 = 6-5. In this way, the problem is solved. You can add 1l to the 3l cup and then import 4l to it. This 1l can make a 1l of water or a 1l of space. here we need to make 1l of water.

When solving this problem, we should consider 4 = 2 + 2 to 4 = 1 + 3 or 4 = 5-1. Here we use 1 + 3, you can also use 5-1.