Think of a Number

Here's a trick to try on your friends:

Thinking

Ask them to think of a number,
then add four,
then multiply the answer by two,
then take away six,
then divide the answer by two,
then add six
and take away the number they first thought of.

Now amaze them by telling them the answer is seven!

 

Numbers

How does this trick work?
It's easy to see when you use algebra:

Ask them to think of a number,
 

 Call this number x

then add four,
 

 Now you have x + 4

then multiply the answer by two,
 

 That makes 2x + 8

then take away six,
 

 That leaves 2x + 2

then divide the answer by two,
 

 x + 1

then add six
 

 x + 7

and take away the number they first thought of.

 That leaves 7!

Once you have understood how this trick works you could try to make up a 'think of a number' trick of your own.

Click here to go back to the Mathematical Magic page

 

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