Consider the distribution below:

Salary (in Rs.)	Number of employees
More than or equal to zero	51
More than or equal to 10,000	48
More than or equal to 20,000	37
More than or equal t0 30000	25
More than or equal to 40,000	19
More than or equal to 50,000	9

 

The frequency of class intervals $30000-40000$ is as follows:

Frequency of any class = cumulative class frequency the cumulative frequency of the previous class

The cumulative frequency is the sum of all previous frequencies up to this point.

The cumulative frequencies are already available. Let us investigate the frequencies:

Hence, the frequency of class intervals 30,000-40,000 is 6 .