Draw a frequency polygon for the following frequency distribution:

Class	$20-30$	$30-40$	$40-50$	$50-60$	$60-70$	$70-80$
Frequency	$5$	$7$	$12$	$7$	$10$	$9$

(N/A) To draw a frequency polygon,follow these steps:
$1$. Find the class marks (mid-points) for each class interval using the formula: $\text{Class mark} = \frac{\text{Lower limit} + \text{Upper limit}}{2}$.
$2$. Calculate the class marks:
- For $20-30$: $\frac{20+30}{2} = 25$
- For $30-40$: $\frac{30+40}{2} = 35$
- For $40-50$: $\frac{40+50}{2} = 45$
- For $50-60$: $\frac{50+60}{2} = 55$
- For $60-70$: $\frac{60+70}{2} = 65$
- For $70-80$: $\frac{70+80}{2} = 75$
$3$. To close the polygon,include two additional classes with frequency $0$: $10-20$ (mid-point $15$) and $80-90$ (mid-point $85$).
$4$. Plot the points $(15, 0), (25, 5), (35, 7), (45, 12), (55, 7), (65, 10), (75, 9), (85, 0)$ on a graph paper where the $x$-axis represents the class marks and the $y$-axis represents the frequency.
$5$. Join these points with straight line segments to form the frequency polygon.