Problem #76: Magic Numbers
This is a good way to mystify a friend. Ask someone to pick a number,
any number at all, but not to tell you what it is.
Have him silently multiply it by five, add five to the product,
multiply the resulting number by two, add two to it, and tell you the result.
Instantly you are able to tell him the number he started with.
What is the secret and why does it work?