The ratio of the area of a circle to the area of a square in which the circle is inscribed is . Hence, the area of the square that is NOT included in the circle (i.e. the "waste") is times the area of the square. Since your large piece of cardboard has dimensions that are both evenly divisible by the diameter of the circles, namely 6, you need only calculate times the area of the 42cm by 36cm cardboard.
John
My calculator said it, I believe it, that settles it