If the average marks of three sections of Class IX, having 30, 45 and 50 students, respectively, is 80, 75 and 90, then the average marks of all the students of Class IX, is:

Correct option is B

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Given:
Number of students in three sections = 30, 45, 50
Average marks of three sections = 80, 75, 90
Formula Used:
Average = Total Marks $$\div$$​ Total Students
Solution:
Total marks of first section = 30 × 80 = 2400
Total marks of second section = 45 × 75 = 3375
Total marks of third section = 50 × 90 = 4500
Total marks of all students = 2400 + 3375 + 4500 = 10275
Total number of students = 30 + 45 + 50 = 125
Average marks of all students = 10275$$\div$$​ 125 = 82.2