Turn all cards over so that you cannot see the numbers. Ask someone to pick 1 card of each color and put them in a row, face down..
Then ask them (or a different person) to pick from the remaining cards and do the same, ie, select one card from each color and put them in a row below the previous 3 chosen in the same color order.
Repeat this process 1 more time.
You will now have 3 sets of cards of 3 cards. There will be 3 cards left over, which you can remove from the playing field.
While your back is turned, have them have them turn the cards over. Each of these sets creates a 3 digit number. Have them add the 3 numbers together and write that number down. At the same time, you write down a number on a piece of paper.
The number you have written will match their number exactly.
Pretty amazing and easy to do. But, very difficult for anyone to figure out how you did it.
Copyright Creative Crafthouse, made in Florida
Please watch the video for details.