If U, the universal set, is the set of all students of a school which are in classes from I to X and B = set of all students of class X.
What is B'?
set of all students from classes I to IX
U = set of all students from classes I to IX
The complement of a subset B of universal set U is the set of all elements of U which are not the elements of B .
Hence B' = set of all students from classes I to IX