A Funny Expressions

This expressions can calculate your heart and your age. It's very veracious.
First, you choose a number in 0,1,2,3,4,5,6,7.
(Your selective number×2＋5）×50
If your birthday has passed this year, then plus 1757
If your birthday hasn't passed this year, then plus 1756
Finally, subtract your natal year.
The first number of the result is the number you choose, the last two numbers are your age.

For example, I choose 4, (4×2＋5）×50=650
I hasn't pass my birthday this year,then 650+1756=2406,
I born in 1986, 2406-1986=420,
Kiko is choose 4, and I am 20 years old. It's very veracious.