First we know that a circle is 360 degrees. From the center of the pentagon if you draw a segment to each point M N P Q R, you create 5 triangles from the center. So 360/5 = 72 degrees. Each 'turn' from letter to letter is a 72degree rotation about the center O.
Moving N 72 degrees maps it onto M. Moving N 144 degrees maps it onto P. Mapping P to Q would be a 72 degree rotation since they are one rotation away from each other. I hope you understand.