Explanation

First, we arrange the marks in ascending order and prepare a cumulative frequency distribution table.

Here n=100, which is even
`therefore` Median will be the average of `50^(th)` and `51^(th)` observations.
`because` `50^(th)` observation is 28
`51^(st)` observation is 29
`therefore` Median =`(28+29)/2=28.5`