Here's a really easy Math trick you can impress your friends and family with. All you need is 4 index cards and a pen. Here's what you do:
- Copy what's on the four cards on the right to your four index cards.
- Ask your friend to think of a number between 1 and 15.
- Show your friend all 4 cards.
- Ask your friend to tell you which cards the number appears in.
- Count 1 if he tells you card 1,
2 if he tells you card 2,
4 if he tells you card 3, and
8 if he tells you card 4
- Add up the counts.
For example, if your friend thinks of the number 6, then he'll tell you it appears on cards 2 and 3. Card 2 counts as a 2 and card 3 counts as a 4. Add up the counts, 2+4, and you get 6, the number your friend thought of.
Another example, if your friend thinks of the number 9, then he'll tell you it appears on cards 1 and 4. Card 1 counts as a 1 and card 4 counts as an 8. Add 1+8 and you get 9.
Can you figure out why this trick works? Hint: It has to do with the binary number system.