Sometimes we don't have all the right number candles to put on birthday cakes... so sometimes you have to read an equation.
As you can see below, the large '1' and '5' are actually 15. You will note the small '2,' small to denote a subtraction. So 15-2= 13.
Happy 13th birthday, Sean. Now that you're old and all, quit picking bundt cakes for your birthday, Dweeb.