Convert the mid points as class intervals: the common difference between the mid points is 7, half of it is 3.5. Subtract 3.5 in all midpoints to get lower limits and add 3.5 to all upper limits to get upper limits.
To get mode (Z) join the upper vertices of the tallest rectangle to their opposite upper vertices of the rectangles adjacent it. Then from the point of intersection of these diagonals draw a perpendicular line to the x-axis, The point at which the perpendicular line touches the x-axis, gives the value of mode.
