In a class the average marks obtained in a science test by a group of 12 students is 70, by another group of 15 students is 85 and that by another group of 18 students is 90. Find the average marks of all the students.

Correct option is C

Given:

Group 1: 12 students, average = 70

Group 2: 15 students, average = 85

Group 3: 18 students, average = 90

Formula Used:

Overall Average = $$\frac{\sum (\text{Number of students} \times \text{Average})}{\text{Total number of students}}$$​​

Solution:

Total marks = 12 × 70 + 15 × 85 + 18 × 90

= 840 + 1275 + 1620 = 3735

Total students = 12 + 15 + 18 = 45

Average marks of all students = $$\frac{3735}{45}$$​ = 83