Explanation:

 No need to remember any formula to solve this type of question. Instead try to realise the things with a simple and small example. Now before moving on to the solution, let’s try to realise this with another small example.
Expand the five fingers of your left hand facing towards you and then rotate it in a 90 degree clockwise manner. Now, you start counting from the top and stop at position of 2nd finger. Now start counting from the bottom till reaching the same finger and you will stop counting with number 4.
That means in this type of arrangement, if the position of the certain thing is X among Y group of members from the top, then its rank from the below will be (Y - X) + 1.
Now apply the same logic over the above question. The rank of Hari is 7th from the top. Total number of students is 30. Now the rank of Hari from the bottom will be (30 - 7) + 1 = 24.
Therefore, the total number of students in the class is 46.