Cell Phone Math Number Trick

Do you know someone who has a cell phone? duh! yeah! Well here's a cool cell phone math number trick!

• Grab a calculator (you won’t be able to do this one in your head)
• Key in the first three digits of your phone number (NOT the area code)
• Multiply by 80
• Add 1
• Multiply by 250
• Add the last 4 digits of your phone number
• Add the last 4 digits of your phone number again
• Subtract 250
• Divide by 2

Recognize the number you end up with? It should be the phone number with all the digits.

Math Behind the Trick

In Step 1, the first three digits of the phone number are multiplied by 80. So if we let x equal the first three digits, it can be written as:

80x.

In Step 2, 1 is added to the result from the previous step. So let's write the algebraic expression for that:

80x + 1

Step 3 tells us to multiply the previous rest by 250. This will look like this:

250(80x + 1)

Step 4 tells us to add the last four digits of the phone number. So we will let y equal the last four digits.

250(80x + 1) + y

The next Step tells us to Add the last 4 digits of your phone number again. So how do you think this should look?

That will look like this:

250(80x + 1) + y + y.....since we can add the like terms together, this simplifies to:

250(80x + 1) + 2y

In the next Step we are suppose to Subtract 250.

So this will give us:

250(80x + 1) + 2y - 250

The last Step tells us to Divide by 2. So, that will be:

[250(80x + 1) + 2y - 250] / 2

Now we will do a little more algebra here and factor a 2 out of the numerator.

2[125(80x + 1) + y - 125]/2

Now, the 2's in the numerator and denominator cancel out to give:

125(80x + 1) + y - 125

Now we can distribute the 125 across the parentheses to get:

10000x + 125 + y - 125

Since 125 - 125 = 0 we will have:

10000x + y

So When you multiply the first 3 digits by 10000 you will have xxx0000 and then you add the last four digits to end up with xxxyyyy, your telephone number! Therefore it should work no matter what the phone number.

Math number tricks are good for practicing addition, subtraction, multiplication, and division.

Students have fun doing the math number tricks, but at the same time they are getting better at doing the math. Make sure they don't use a calculator...sort of defeats the purpose.

Most of us have a pack of cards laying around the house somewhere. Check some cool Math Card Tricks here. Otherwise click the link below to go to the main math tricks page.