There are 100 students in 3 sections A, B and C of a class. The average marks of all the three sections was 84. The average of B and C was 87.5 and the average marks of students in A section was :

[A]20

[B]25

[C]30

[D]35

**20**

Numbers of students in section A = x

∴ Numbers of students in section B and C = (100 – x)

∴ x 70 + (100 – x) 87.5 = 84 100

=> 70x + 87.5 100 – 87.5x = 8400

=> 8750 – 17.5x = 8400

=> 17.5x = 8750 – 8400

Hence option [A] is correct answer.