How many student are there in the class?
I. there are more than 20 but less that 27 student in the class .
II. there are more than 24 but less then 31 student in the class the number of student in the class can be divided into group such that each group contains 5 students.

From Statement I,
Number of student in the class
= 21 or 22 or 23 or 24 or 25 or 26
From Statement II,
Number of student in the class
= 25 or 26 or 27 or 28 or 29 or 30
Sine , number of student can be divided into group of 5 students, therefore number of student must be multiple of 5.
? NUmber of student in the class = 25 or 30
Combining Statement I and II .
Number of Student in the class = 25
Hence , the data in both the Statement I and II together are necessary to answer the question .