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

A. 12
B. 25
C. 34
D. 67

Total marks of students = 80 x.
Total marks of (x – 5) students = 90(x – 5)
80x – (5 * 40) = 90(x – 5)
250 = 10x => x = 25