The average marks of a class of 30 students is 40 and that of another class of 50 students is 60. Find the average marks of all the students?

A. 50
B. 47.5
C. 59
D. 52.5
Sum of the marks for the class of 30 students = 30 * 40 = 1200
Sum of the marks for the class of 50 students = 50 * 60 = 3000
Sum of the marks for the class of 80 students =
1200 + 3000 = 4200
Average marks of all the students = 4200/80 = 52.5