Number of student with thesis page 0-9--1
Number of student with thesis page 10-19--4
Number of student with thesis page 20-29--6
Number of student with thesis page 30-39--7
Number of student with thesis page 40-49--2

so if we form a series for number of pages it will have 1+4+6+7+2=20 terms..median will be 20/2th and 20/2+1 th terms..or
average length of the 10 and 11 tth student=1/2(length of 10th student+length of 11th student)
11th student is in the group of 20-29 pages and median<length of 11th student

since no other student in the group after 11th student next student is in group of 30-39..at least one will have the least value 30 here...so the least number of student whose thesis were within six pages of the median length is 11th student and 12th student or 2

Hope this will clarify the doubt.