Answer:
 3
Explanation:
Those wearing a shirt of another color are ...
 1 - 5/8 -1/4 = 8/8 -5/8 -2/8 = 1/8
of the total number of students in the classroom
 (1/8)×(24 students) = 3 students . . . . wearing another color
_____
Alternate solution
With the given information, you know ...
 (5/8)(24) = 15 . . . students wear blue
 (1/4)(24) = 6 . . . . students wear white
 24 -15 -6 = 3 . . . students wear another color