I'll Guess Your Age

8 x 2=16, + 5 =21, x 50=1050, add 1768 =2818, subtract from year of birth, using just last two numbers of that result as my age produces a number that is off by 51 years. We are being trolled even if OP is sincere. The premise is junk.

Let's solve this with maths!
Step 1: x
Step 2: 2x
Step 3: 2x+5
Step 4: 50(2x+5)=100x+250
Step 5: Assume we have already had our birthday (2019 considered)
100x+2018
Step 6: Subtract birthyear. So, the birthyear is equal to 2019 (we have already had our birthday) subtract our age. 2019-y. (y is age)
So, 100x+2018-(2019-y)
= 100x-1+y.
Therefore this works if you are under the age of 100; you get your first digit as you original number x, followed by y, your age. eg you get the number QRS. Q is your number, and RS is your age; if your age is one digit, R will be a zero.
Unfortunately, this "trick" is one year out of date (hence the -1 in the equation).

Your math is wrong? Favorite number = 1 . Got 2118-1980(when I was born, already had b-day this year) = 138. So 38 should be my age according to you, but I'm 39.
you don't talk like 39
Choose a number from 1 to 9 which is your favourite number
Multiply your number with 2
ADD 5 YOUR RESULT
Multiply the result by 50
If you already celebrated your birthday this year add 1768 to your result
OR
Your birthday is coming add 1767
the first number you chose are nothing and 2 digot
You will have 4 digit number if not then something gone terribly wrong
You need to substract the year from your birthyear
the first number you chose are nothing and after 2 digits are your age