In a class of 90 students, the marks obtained in a weekly test were as under :

Marks	No. of students
16 - 20	4
21 - 25	12
26 - 30	18
31 - 35	26
36 - 40	14
41 - 45	10
46 - 50	6

Draw a frequency polygon for the above data.

The following frequency distribution is discontinuous, to convert it into continuous frequency distribution,

Adjustment factor = (Lower limit of one class - Upper limit of previous class) / 2

= $\dfrac{21 - 20}{2} = \dfrac{1}{2}$

= 0.5

Subtract the adjustment factor (0.5) from all the lower limits and add the adjustment factor (0.5) to all the upper limits.

Continuous frequency distribution for given data is :

Steps to draw frequency polygon :

1. Since, the scale on x-axis starts at 10.5, a kink is shown near the origin on x-axis to indicate that the graph is drawn to scale beginning at 10.5.

2. Take 1 cm along x-axis = 5 marks.

3. Take 1 cm along y-axis = 5 students.

4. Find the mid-points of class-intervals.

5. Find points corresponding to given frequencies of classes and the mid-points of class-intervals, and plot them.

6. Join consecutive points by line segments.

7. Join first end point with mid-point of class 10.5 - 15.5 with zero frequency and join the other end with mid-point of class 50.5 - 55.5 with zero frequency.

The required frequency polygon is shown alongside.