Amaze your friends

     Amaze your audience by working out not only their age but also what size shoe they wear! Wow them even more by telling them how the maths works.
Give them the following directions but tell them not to show you any calculations:
•    Write down your age.
•    Multiply it by 1/5 of 100.
•    Add on today's date (e.g. 2 if it's the 2nd of the month).
•    Multiply by 20% of 25.
•    Now add on your shoe size (if it's a half size round to a whole number).
•    Finally subtract 5 times today's date.
•    Show me you final answer!
Look at the answer, the hundreds are the age and the remaining digits are the shoe size. If for instance somebody shows you 1105, there are 11 hundreds - the age, and the remaining digits 05 (or 5) show the shoe size.
Now, how on earth does that work?