Because the number of students is an integer, it must be a common multiple of 5, 3, 12, and the minimum common multiple is 60. There are not enough students in class 6 13, so it is 60.

60* 1/5= 12 people, full score, 60* 1/3=20 people with 80 points or more, 60*5/ 12=25 people passed.

60- 12-20-25=3 people failed.

This description is a bit vague. Does the pass include full marks and above 80 points?