Take your calculator, or your pencil
Put on your shoe size (no half size)
multiplie by 5,
multiplie the total by 20,
adds (current year - 1000),
subtract the year of your birth.
Now you have a number with 3 or 4 digits (depending on the size standard used, either on 1 or 2 digits)
The first/the first ones give(s) you your shoe size,
The last 2 give you your age !!!
Friends mathematicians, thank you for explaining me the trick!