Maybe you didn't follow instructions Kota

I've donde this more than ten times and it always returns the right answer...

For instance:

I chose the following number 7699, jumbled --> 9796

subtraction: 9796 - 7699 = 2097

I circled number nine, jumbled ----> 702

Typed in 702 and fido answered 9

I guess the addition of your last number gives out the number you circled.

Let me see that again:

I chose the following number 3865, jumbled --> 8365

subtraction: 8365 - 3856 = 4509

I circled number five, jumbled ----> 094

Typed in 94 and fido answered 5!

Ok now I see, when the addition of your last number is greater than 9, then it subtracts. 9 - 4 = 5

Ok so what if you end up with the same digit (except for the zero of course)

1478, 4781, 4781 - 1478 = 3303, you should enter 303, 330, 33

fido's answer will be 3 always.

What if you end up with a 3 digit number with no zeros in it?

9732 - 2379 = 7353

circled number 5, jumble ---> 337

If I enter 357 then fido's answer is 3....I can't see the pattern

Ok I need some help with this

So cracking the code, so far

- remove the zeros
- if digits are all the same

- then output: the repeating digit
- else somewhere in here we must deal with the number 3,3,5,7 lol

- sum = sum of all digits
- if sum < 9

Keep trying