In such series there is a slight change in the formula for calculating median. Since the series are cumulated in descending order the cumulative frequency of the class preceding the median class is found out by subtracting the cumulative frequency of the median class from the total of the cumulative frequency. In order words :                

c = (N-c.f.)

where,       c      = cumulative frequency of the class proceeding the median class

                N      = total cumulative frequency

               c.f      = cumulative frequency of the median class.