In one empty room there is only 500 CD on the floor. Each CD, from one side is red and the other green. The 100 CD is red (obviously the other 400 is green). In the room there is no light.
You have to find a way to create two groups (no need to be equal in number), which both have the same number of red CD.
We divide them into 2 groups. One of 400 and one of 100 cd. Then we turn it upside down all the cd's in the group of 100. If in this example we have 20 red cd then in the other will be 80 cd and by inverting them, twe will have 80 cd too.