The average marks obtained by a class in an examination were calculated as $30.8$. However, while checking the marks entered, the teacher found that the marks of one student were entered incorrectly as $24$ instead of $42$. After correcting the marks, the average becomes $31.4$. How many students does the class have?

Let the number of students in the class be $n$.The initial calculated average is $30.8$.Initial total marks = $30.8n$ The marks were entered as $24$ instead of $42$.The correct total marks should be the initial total minus the incorrect marks plus the correct marks:
$$\text{Correct Total}=30.8n-24+42=30.8n+18$$
We are given that the new (corrected) average is $31.4$.Therefore, we can set up the equation:
$$\frac{30.8n+18}{n}=31.4$$
$$30.8n+18=31.4n$$
$$31.4n-30.8n=18$$
$$0.6n=18$$
$$n=\frac{18}{0.6}=30$$
The class has $30$ students.