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Black Cards

Preparation:
Cut out five small cards.
On one side they are numbered 1,2,3,4,5 in RED.
On the other side they are numbered 6,7,8,9,10 in BLACK.
Make sure that the "6" is on the reverse of the "1" and the "7" is on the reverse of the "2" and so on.

Procedure:
Pass the five cards to a friend.
You will do this trick without looking at the cards!
Ask your friend to shuffle and place the five cards on the table.
Ask your friend to tell you how many Black cards are facing upwards.
Before your friend has finished - you can shout out the answer!

The Secret:
If your friend says there are no black cards, the total will be 15.
(Because if all the Red numbers are showing the total must be 1+2+3+4+5 = 15.)
If there are any black cards showing, you add an extra 5 for every black card.
Example: If there are 2 black cards, you add 2 x 5 = 10.
This would make the total 10 + 15 = 25.
It would not matter which two cards are Black!
If all the Black cards were showing, the total would be 15 + 25 = 40.

PS: Explain how this is possible.

Watch the video.

 PS : Try this other card trick too!