The average marks of three classes of 50, 65 and 45 students, respectively, are 75, 80 and 90. Find the average marks of all the students.

Correct option is B

Given:

Average marks for 50,65 and 45 students = 75,80 and 90 respectively

Formula Used:

​$$Average =\frac{Sum\ of \ observations}{Number\ of\ observation}$$​​

Solution:

Total marks of 50 students = 50 $$\times$$​ 75 = 3750

Total marks of 65 students = 65 $$\times$$​ 80 = 5200

Total marks of 45 students = 45 $$\times$$​ 90 = 4050

Total marks of all students = 3750 + 5200 +4050 = 13000

Total number of students = 50 +65+45 = 160

Average = $$\frac{13000}{160} = 81.25$$​​