Games
Problems
Go Pro!

Congratulations! You've found one of the Easter Egg pages at The Problem Site! As you browse the site you may see random bits of text or little icons that lead to one of our secret easter eggs. Happy hunting!

# The Telephone Game

You can impress your friends with this calculator game; you can impress them even more if you can explain to them how it works!

Grab a calculator.
Type in the first three numbers of your seven digit telephone number.
Multiply by 80
Multiply by 250
Subtract 250
Divide by 2

What is the final number?

The result, of course, is your telephone number. Seems bizarre? Not really. If you know some algebra, keep reading to find out why it works!

Let X be your first three digits, and Y be your last four digits.

Then the steps are as follows:

Begin with X
Multiply by 80 --> 80X
Add one --> 80X + 1
Multiply by 250 --> 20000X + 250
Add Y (twice) --> 20000X + 250 + 2Y
Subtract 250 --> 20000X + 2Y
Divide by 2 --> 10000X + Y

Which, of course, is your telephone number.

# Featured Games on This Site

Match color, font, and letter in this strategy game
Trap all the dots in this problem-solving puzzle

# Blogs on This Site

Reviews and book lists - books we love!
The site administrator fields questions from visitors.