While calculating the average marks of a class, the score increased by 0.5 when marks of one student is wrongly entered as 54 instead of 45. The number of students in the class is:

Correct option is B

The incorrect mark entered was 54 instead of 45.

$\text{Difference}=54-45=9$

Since the average increased by 0.5 when this difference was added to the total marks,

n = $$\frac{9}{0.5} = 18$$​​