Solutions
Given, Number of students in section A, B and C are 105, 63 and 42 respectively and average marks of section A, B, and C in an exam are 63, 67 and 58 respectively.

⇒ Sum of observations = Average × Total number of observations
So, Sum of marks of all students of section A of class 10th =105 × 63 = 6615
And Sum of marks of all students of section B of class 10th = 63 × 67= 4221
And Sum of marks of all students of section C of class 10th = 42 × 58 = 2436
Total marks scored by class 10th = 6615 + 4221 + 2436 = 13272
Total number of students = 105 + 63 + 42 = 210
So, Required average =
= 63.2