Scope the question with the interviewer:

- Taking India's air-space as a use-case

First, let's try and estimate the average number of hours a plane is in the air.

My list of assumptions:

- Assuming a single average trip time of the plane is 3 hours which includes on an average 2 hours in the air, and 1 hour on the ground ( for turnaround - ground operations / deboarding & boarding).

- assuming a plane remains in operation for around 22 hours, since there are no operations between 2 - 4.

- for International Flights, it takes 2 hours to move out of Indian air space.

Basis above assumptions, we can assume, out of 24 hours, a plane is in the air for a total of 22*2/3 ~ 15 hrs hours -> probability of plane being in air ~ 0.6

Next, let's calculate the total number of flights. Assuming there are 6 major airports, 20 medium airports, and 30 smaller airports. Assuming, there are flights at every 2 mins in major airports, every 15 mins in the medium airport and every 30 mins in small airports (average flights adjusted for both peak and non-peak hours).

Total flight = 22 (total operational hours in a day) * (6*60/3 + 20*60/15 + 30*60/60) ~ 5060

Total flight in air = Total flight * probability of flight being in air = 5060 * 0.6 ~ 3035