Solve this:
8.The following table  gives the frequency distribution  of marks obtained by 199 students in an examination:

Marks	No. of students
0-20	21
21-30	19
31-40	60
41-50	42
51-60	24
61-70	18
71-80	15

Find the percentage of  failure if minimum marks for passing the  examination is 35.

Dear student, 
The given frequency distribution is an inclusive series. It can be converted into an exclusive series as follows:
 

Class Interval	Frequency
-0.5 - 20.5	21
20.5 - 30.5	19
30.5 - 40.5	60
40.5 - 50.5	42
50.5 - 60.5	24
60.5 - 70.5	18
70.5 - 80.5	15

In a grouped frequency table we assume that the frequencies are evenly distributed. Accordingly, for the class interval (30.5 - 40.5) we can assume that half of the frequency (i.e.30) lies in (30.5 - 35.5). 
So, the total number of students who have marks less than 35.5 are 21 + 19 + 30 = 70. This is equal to 35.2% of the total number of students (i.e. 70/199)