Correct Answer - Option 2 : 79.33

**Given:**

80 is the average marks of 4 students

90 is the average marks of 5 students

70 is the average marks of 6 students

**Formula:**

Average marks = total marks/ total number of students

**Calculation: **

If 80 is the average marks of 4 students then 80 × 4 = 320 is the total marks of 4 students.

If 90 is the average marks of 5 students then 90 × 5 = 450 is the total marks of 5 students.

If 70 is the average marks of 6 students then 70 × 6 = 420 is the total marks of 6 students.

Now the total number of students = (4 + 5 + 6) = 15

Required average = (320 + 450 + 420)/15

⇒ 1190/15 = 79.33

**∴**** The average marks of all the students is 79.33**