13 Years Old Algebra Equation Funny 13th Birthday Math Shirt Solid colors: 100% Cotton; Heather Grey: 90% Cotton, 10% Polyester; All Other Heathers: 50% Cotton, 50% Polyester Imported Machine wash cold with like colors, dry low heat Remember PEMDAS from math class? You'll need it for this shirt. Great birthday gift for math lovers Lightweight, Classic fit, Double-needle sleeve and bottom hem