Number Magic

Do the following:

1. Think of a number.
2. Multiply it by 3.
3. Add 4
4. Multiply it by 2.
5. Subtract 2.
6. Divide it by 3.
7. Add 4.
8. Divide it by 2.
9. Take the number you have think of minus the final answer.
10. Is your answer 3?

This works for every number (including infinity!). Some of you might have known the trick behind it, but for others who are curious, read on.

And the secret formula is, ALGEBRA!!!

Look at how algebra can be used to solve this:

1. Think of a number. (x)
   
2. Multiply it by 3. (3x)
   
3. Add 4 (3x+4)
   
4. Multiply it by 2. (6x+8)
   
5. Subtract 2. (6x+6)
   
6. Divide it by 3. (2x+2)
   
7. Add 4. (2x+6)
   
8. Divide it by 2. (x+3)
   
9. Take the number you have think of minus the final answer. (3)
   
10. Is your answer 3? (Yes!)

This is actually the trick behind all of these number magics. All they do is to make it so complicated that you are amazed by the "magic" behind it, and when they say, "there is a secret formula behind it...", you will be so amazed that you keep thinking about it all they long.

There are many methods of presenting this magic trick. Take for example that I'm testing Sheng Yao:

Me: Think of a number.
S.Y.: Yep.
Me: Multiply 4.
S.Y.: Yep.
Me: Add 10.
S.Y.: Yea...
Me: Divide by 2.
S.Y.: Um.... ok.
Me: Add 5.
S.Y.: Ok.
Me: Divide by 2.
S.Y.: Yep.
Me: What is the number?
S.Y.: 15.
Me: Is your original number 10?
S.Y.: How did you know?

This is a simple trick. Now let us work it out.

1. Think of a number. (x)
2. Multiply 4. (4x)
3. Add 10. (4x+10)
4. Divide by 2. (2x+5)
5. Add 5. (2x+10)
6. Divide by 2. (x+5)
7. What is the number? (x+5)
8. Is the original number ____? (Just take the number that he gave you in Step 7 and subtract it by 5!)

Simple, isn't it?

There are many other ways to present this trick, so I wish to discuss this with you! Just leave a comment in the comment box, or send us an email!