to see the source of this problem and the solution:
it's an old book of mathematic that i've found on Gutenberg Project.,
page-17
A Scrap-Book of Elementary Mathematics: Notes, Recreations, Essays by White
A person privately fixing on any number, to tell him that number.
After the person has fixed on a number, bid him double it and add 4 to that sum,
then multiply the whole by 5; to the product let him add 12, and multiply the amount by 10.
From the sum of the whole let him deduct 320, and tell you the remainder,
Now create a program to find the mysterious number ?
and the general math formula behind it !
Here is the book where i found this:
