Can you turn this into an Algebraic Explanation?

I don't know if this is considered advanced/uni math so sorry in advance if this is in the wrong section, but here's my problem:

There is a trick that I found over the internet and I was given the challenge of making it into an Algebraic Explanation.

Here's the trick

1: Grab a calculator (You wont be able to do this one in your head) .
2: Key in the first three digits of your phone number (NOT the area code-if your number is 01-123-4567, the 1st 3 digits are 123).
3: Multiply by 80.
4: Add 1.
5: Multiply by 250.
6: Add the last 3 digits of your phone number with a 0 at the end as one number eg, 5670
7: Repeat step 6
8: Subtract 250
9: Divide number by 20

I was given

Quote:

the number you start with, 123 or whatever, can be thought of as 100x1 + 10x2 + 3

as a hint.

I've tried playing around with it, giving the numbers variables etc but I still can't turn it into an explanation - I don't even know how to turn it into an explanation.

Thanks in advance