I did not see any possibility about this method to guess your missing number
say, you have a phone number as AXB, while A is the first 3 digits, X is the missing one, B is the last 4 digits.
1- enter the first three numbers of the phone
2- multiply by 80
3- add 1 to that total
4- multyply by 250
5- add four last digits of your phone two times
6- subtract 250 from total
7- divide by 2
(((A * 80 + 1) * 250 + B * 2) - 250) / 2
= A * 80 * 250 / 2 + B
= A * 10000 + B
so nothing related to X
You need some kind of rule that all phone numbers follow in order to get the missing digit. For example 'all phone numbers total 47' or 'all phone number are divisible by 11'.
At the moment (as briefly explained above), all you're getting is:
2) Multiply by 80 - We will do this together with 4) Multiply by 250. These two steps together are equivalent to doing n * (80 * 250) or n * 20000.
3) +1 - Since this is getting multiplied by 250, you're just adding 250 to the number.
6) Subtract 250 from your number - This simply undoes step 3. So step 3 and 6 can be ignored, as they have no overall effect.
5) Lets call the last 4 digits 'p' and so we add 2p.
7) Divide by 2 and we are left with (20000n + 2p) / 2 or simplified: 10000n + p.