Mind reading with binary numbers

[Mike62]

Hi..

Here is a very simple mathematical trick you can play with binary
numbers. Cut two plain postcards (or sheets of paper) in half and
label the four pieces A B C and D. Mark numbers on each card like
this:

A B
8 9 10 11 4 5 6 7
12 13 14 15 12 13 14 15

C D
2 3 6 7 1 3 5 7
10 11 14 15 9 11 13 15

Give the four cards to a friend. Ask him/her to think of any number
between 1 and 15 and tell you the cards on which the number he/she
thought of appears. You can then tell him/her the number they thought
of.

To be able to do this, write down a 1 for each card that carries his/
her number, and a 0 for the other card or cards, keeping the order A
B C and D. What you have written down will be the binary number of
the number chosen. For example, suppose he/she chose 10. You would be
told that the number appeared on A and C. You would write down:

A B C D
1 0 1 0

This is the binary number for 8 + 2 = 10. It works every time.

Best wishes

Mike