Answer:
N = number of students
Half of the students of a class are dancing
D = ½
3/4 of the remaining play cricket
C = ¾(½N)
Y = students studying in the library
Y = ¼(½N)
If Y = 9,
¼(½N) = 9
½N = 9 * 4
½N = 36
N = 36 * 2
N = 72
There are 72 students in the class.