The given table can be written as:
 $$\begin{array}{cccccc}\text{Marks}& 0-10& 10-20& 20-30& 30-40& 40-50\\ \text{No. of students}& 5& 5& 13& 8& 9\\ \text{Cumulative Frequency}& 5& 10& 23& 31& 40\end{array}$$
No. of students can be calculated as follows:
5
10 - 5 = 5
23 - 10 = 13
31 - 23 = 8
40 - 31 = 9
$\therefore \frac{n}{2}=\frac{40}{2}=20$
Since there are a total of 40 observations, the median class is the one whose cumulative frequency is closest to and greater than 20.
Here the cumulative frequency for the class interval '10 - 20' is 23 which is greater than 20.
The class interval 20 - 30 is the required answer.