Draw more than Ogive curve for the following data showing the marks secured by the students of Class XI in a school.

Marks	Number of students
$0-10$	$2$
$10-20$	$4$
$20-30$	$9$
$30-40$	$10$
$40-50$	$8$
$50-60$	$5$
$60-70$	$3$
$70-80$	$2$

Find median of the data.