Answer:
18 students
Explanation:
Assuming there are 21 students in the class,
3 of them slept for 12 - 15 hours
8 of them slept for 8 - 11 hours
8 of them slept for 4 - 7 hours
2 of them slept for 0 - 3 hours
Now we just add the number of students that slept under 12 hours
8 + 8 + 2 = 18 students
Hope this helps!