A Mathematical Equation

You know how there are those equations where you take a number, do mathematical stuff to it and it always works out? Like this: Think of a number. Double it. Add six. Half it. Take away the number you started with. The answer is always 3.
 
I have one of those. But it's a little more advanced. Don't feel bad if you have to use a calculator.

Take your age - subtract 2. Add 2. That is your age.

It works every time too. 
 

This is actually true.  I have eaten a lot of chocolate and I can count 15 pieces in the picture.