Good eyes Mikaztro.

If you guys remember your algebra: here's the analogy.

X = first 3 numbers of your phone number (ex. 555)
Y = last 4 numbers of your phone number (ex. 1234)

1. X
2. X * 80
3. 80X + 1
4. (80X + 1) * 250
= 20,000X + 250
5. 20,000X + 250 + Y
6. 20,000X + 250 + Y + Y
7. 20,000X + 2Y + 250 -250
8. (20,000X + 2Y) / 2
= 10,000X + Y
= 10,000 (555) + 1234
= 55,000 + 1234
= 555.1234