The average marks of a group of 30 students in a class reduces by 2 when a good student who scored 90 marks leaves the class and a new student joins the class. Find the marks of the new student.

The new student scored 30 marks.

The class size stays 30, and the average falls by 2.

So the total falls by 30 x 2 = 60.

The leaving student had 90, so new - 90 = -60.

Hence the new student's marks = 90 - 60 = 30.