Q:8 (B)
My Ans: Number of students at school = 1200

HOW?
I sort of did it with my own reason & logic, the answer is correct but the method is a bit crude I believe, and I solved it in the paper with following reasoning.

After 20 days.
Before arrival of new batch (of 400 students)= 32 days
After arrival of new batch (of 400 students)= 24 days

Initial Rate of food consumption = 1/32 units/day
After arrival of 400 students, i.e. Final Rate of food consumption = 1/24 units/day

Finding the change of consumption:
Change = Final Rate/Initial Rate = (1/24) / (1/32) = 1.33

=> The consumption of food has increased by 0.33 or 33%, meaning food consumed by extra 400 students.
i.e. 400 students are 33% of the total students added to the school.

If 400 students are 33% of initial number of students, then total students should be 1200. (Do some simple maths now i.e. 400/0.33)