Think:
7:55 a.m. is 5 minutes (5/60 hrs) before 8 a.m.;
from 8 to noon it's 4 hours, and from noon to 2:40 p.m. is 2 2/3 hours.
Thus, you're in school 5/60 hrs + 4 hrs + 2 2/3 hrs, or
6 2/3 hrs + 5/60 hrs, or 6 40/60 hrs + 5/60 hrs, or
6 45/60 hrs, or 6 3/4 hrs.
Of course there are other ways in which you could do this problem:
4 hrs 5 min plus 2 hrs 40 min comes out to 6 hrs 45 min, or 6 3/4 hrs.