Answer:
21/33
Explanation:
1. 8+10+3+12=33 students in school
2. Now, you find how much students play violin or guitar
- 8 students only play violin
- 10 students only play guitar
- 3 students play both
3. 21 students in total who play violin or guitar
4. Since there are 33 students total, 21 out of 33 play those instruments
(If its not right, then adding in the 3 students who play both is probably a mistake :P)