The data is not related to class and the difference of two consecutive values is 5.

∴ To obtain class lengths, I shall take the mid points of the class-interval.

17.5-22.5, 22.5-27.5, 27.5-32.5, 32.5-37.5, 37.5-42.5, 42.5-47.5, 47.5-52.5, for wages 20,25,30,35,40,45,50 respectively.

By joining the middle values of classes, we get the frequency polygon