If the average marks of three sections of Class X, having 35, 40, and 25 students, respectively, are 90, 88, and 80, then the average marks of all the students of Class X, is:

Correct option is D

Given:
Section I → 35 students, Avg = 90
Section II → 40 students, Avg = 88
Section III → 25 students, Avg = 80
Formula Used:
Total Marks = Average × Number of Students
Overall Average = Total Marks of all Sections ÷ Total Students
Solution:
Section I marks = 35 × 90 = 3150
Section II marks = 40 × 88 = 3520
Section III marks = 25 × 80 = 2000
Total marks = 3150 + 3520 + 2000 = 8670
Total students = 35 + 40 + 25 = 100
Overall average = $$\frac{8670} {100}$$​ = 86.7