TITLE: What is the mathematical explanation for this trick?
QUESTION [15 upvotes]: I found the following method to find one's age. It is working exactly for my case. I like to understand and solve this puzzle. If this is wrong forum, my sincere apologies. Please guide me solve this.

Take last digit of your mobile number 
Multiply it by 2
Add 5 with the above answer
Multiply the answer with 50
Add 1764 with the answer
Subtract the your birth year from the result in part 5 (example 1985, 1973, etc.)

Now you will get 3 digit number. The first one is last digit of your mobile number, last two digits are your age.

REPLY [7 votes]: That's elementary. If $d$ is the last digit and $y$ your birth year, you are computing
$$(2d+1)50+1964-y=100d+2014-y.$$
The first term is the digit shifted two positions to the left (times $100$); the other two terms compute your age.
You'll have to increment the constant $1964$ every year, and the trick won't work for centenarians.