Equation = Your Shoe Size and Age! Prince George, BC, Canada / 99.3 The Drive Darren Coogan Blog Jan 19, 2016 | 8:04 AM Here’s that math equation that captivating the internet. Try it out: Shoe size (no half sizes) multiply by 5 = Add 50 = Multiply by 20 = Add 1015 = Subtract birth year = Your shoe size and age!