If I was reading that, I would interpret it as meaning that the 2 larger angles in the diamond should measure 160 degrees each. And if I remember my high school greometry correctly, that means the other 2 angles in the diamond should measure 20 degrees each. In any quadrilateral the total of all the angles = 360, right??
But make sure someone else agrees with or corrects this BEFORE YOU CUT! If it wasn't for a math tutor I'd still be in high school! Boy, did I hate math! :(