Help with math please
I need to make a 12 1/2 inch pinwheel (finished 12 inches) and I would like to make it the way Jenny does. Sewing all four sides of a square, cutting diagonally, etc. I don't know how to figure the math out to make the squares the right size. Can someone give me the formula? Math is not my long suit. Thanks in advance for any help you can give me.