We take imagined class 15-20and 0-55, each with frequency 0. The class marks of above classes are 17.5 and 52.5 respectively.

So, we plot the points A (17.5, 0) and B (52.5, 0). We join A with the midpoint of the top of the first rectangle and B with the mid-point of the last rectangle.

Thus we obtain a complete frequency polygon,