In a class of 30 students, 10 members of Physics main have average of 72 marks, 5 members of Chemistry main have average of 66 marks and 15 members of Mathematics main have average of 80 marks. The average marks of the class is :

Given,

10 members of Physics main have average of 72 marks

$$\Rightarrow$$  Sum of the marks of the students of Physics main = 72 x 10 = 720

5 members of Chemistry main have average of 66 marks

$$\Rightarrow$$ Sum of the marks of the students of Chemistry main = 66 x 5 = 330

15 members of Mathematics main have average of 80 marks

$$\Rightarrow$$ Sum of the marks of the students of Mathematics main = 80 x 15 = 1200

$$\therefore\ $$Average marks of the class = $$\frac{\text{Total marks}}{\text{Total students}}$$

$$=\frac{720+330+1200}{30}$$

$$=\frac{2250}{30}$$

$$=$$ 75