First, let's go through the puzzle to obtain the formula.

1. 1. Take your shoe size 'S' and multiply by 5.

   S × 5

2. 2. Add 50.

   (S × 5) + 50

3. 3. Multiply by 20.

   [(S × 5) + 50] × 20

4. 4. Add 1015.

   [(S × 5) + 50] × 20 + 1015

5. 5. Subtract the year you were born.

   [(S × 5) + 50] × 20 + 1015 - (2015 - A)

6. 6. Now, multiply the equation out.

   (5S × 20) + (50 × 20) + 1015 - 2015 - (-A)

   (5S × 20) + (50 × 20) + 1015 - 2015 + A

7. 7. The numerical terms now cancel out.

   1000 + 1015 - 2015 = 0

8. 8. The formula is now left as...

   100S + A

9. 9. If you substitute the values for S and A.

   (100 × 6) + 55

   = 655

This is my special number.

The first digit is my shoe size 6 and the 2nd and 3rd digits are my age 55.

Why?

It's simple algebra.
Multiply anything by 100 and all the digits in that column(s) move 2 columns to the left.
Then add my age and the magic happens.

Notes
1. If you have already had your birthday in 2016, then add 1 to your answer to give your correct age.
2. This won't work if your are 100 years old or more, but if you've reached that grand age you probably don't care if it works or not!!!

Lots of love

Carol Vorderman