If the sets F and C stand for the sets of students that play football and cricket respectively, then by question
n(F)=250, n(C)=300 and n(F∩C)=120.
But, n(F∪C)=n(F)+n(C)−n(F∩C).
=250+300–120
=430
Thus, the total number of students in the school is 430.