In a music school, three instruments are taught: Tabla, Violin and Guitar. Out of 278 students in the school, 20 learn Tabla and Violin, 23 learn Violin and Guitar and 21 learn Tabla and Guitar. 9 students learn all three instruments. It is known that the equal number of seats in all three instruments classes. (If a student is learning Guitar as well as Tabla, then he occupies two seats: one in Tabla class and one in Guitar class) 
Determine the number of students who have occupied only one seat.