7 Up Read Your Mind

Sandy recently sent out the below link with a pretty obvious challenge to the CA brothers (Steve and I) to explain it. Quote from her email: "so I AM forwarding it to my cousins who ARE mathematical computer software geniuses working for big firms in Silicone Valley"

Here is the link: Fido Puzzle.

I decided to try to figure it out. Did some experimenting this morning for about 10 minutes while watching Looney Tunes on the Cartoon Network (you can be never to old for Looney Tunes). There is some mathematical thing obviously going on, could I determine what it was??

Then it struck me to take Sandy's challenge in a slightly different way. Rather than focus on the mathematical piece, I would focus on the computer software piece.

I know for sure I wouldn't call myself a mathematical computer software genius, but I am very good at finding a quick and easy way to get an answer to a solvable problem.

Solvable problem = find somebody else's answer = Internet = Google (Notice how I made that into a math equation. :) )

Did a quick search via Google: fido puzzle 7 up answer

Which took me to this page (the very first search return result): Have Your Mind Read... which has a good mathematical answer to how the number is determined.

The answer is that the number you get after the subtraction is guaranteed to be divisible by 9. Since the number is divisible by 9, the sum of the digits in the original number needs to sum (add up) to a number that is divisible by 9.

The answer makes sense to me, though I would have never figured it out myself. I did use my "finely tuned" Quality Engineering skills to determine a couple sequence of numbers that won't work. What in the software world is a called a "border test case"

They try to avoid the one sequence of numbers with the words "Make it completely random with lots of different digits". If you were to start with a number such as 111 (a sequence of exactly the same number), you ended up with all zeros (which is not divisible by 9), but the algorithm requires it so the answer generated is 9.

The other scenario that they can't guess correctly is if the number you circled is a 0, so what do they do?, they tell you not to circle the 0. The way the math works out, if you circle the 0, there is two answer to the problem. The answer could be 0 or 9, and they always pick 9.

BTW: You can be too old for "Powerpuff Girls"