The average marks of the students of a class in a particular exam is 80. If 5 students whose average marks in the exam is 40 are excluded, the average marks of the remaining students will be 90. Find the number of students who wrote the exam.

Correct option is A

Given:
Average Marks of Class = 80
5 students excluded those marks are 40, average becomes 90.

Formula Used:

Average = $$\frac{\text{Total Marks}}{\text{Number of students}}$$​​
Solution:
Let the total number of students be N.
Therefore, the total marks for all students = 80 × N.
​Total marks of excluded students = Average of excluded students × Number of excluded students = 40 × 5 = 200.
Remaining marks = Total marks - Total marks of excluded students = 80N - 200.
The average marks of the remaining students is given as 90. Therefore,
​$$\frac {80N - 200}{N - 5} = 90.$$​​
80N - 200 = 90(N - 5).
N = 25
Therefore, the number of students who wrote the exam are 25.