How many people are in the air boarded on a plane at this very moment in India?

The key here is to think about what information you have (or can estimate) which can help you get to the answer. You need to be clear about your logic and assumptions, and be clear-minded enough to be able to sense-check the ultimate answer you arrive at.

First, I clarify the question – what is meant by “airplanes”? All planes, private jets, commercial flights only.

I will assume that “airplanes” can be divided into four categories: commercial passenger, freight, small private and military. There will be other types (like charters and crop-dusters) but to simplify things I will leave them aside.

I assume that these flights are only those flying domestically or to an international destination (but still over India). I will exclude planes flying over India from one country to another. I assume there are:

10 giant airports

20 major airports

50 medium airports

100 small airports

Giant airports would have about 20 flights per hour taking off, 18 hours per day. So, about 360 flights per day. At any one time, perhaps 15 percent of these flights would be in the air. I would assume that the average plane would contain 250 people. So, that’s 360 x 15% x 250 x 10 airports = 135 000 people.

Major airports would have about 10 flights per hour, 18 hours per day. Fifteen percent would be in the air at any one time, with an average of 200 people on board. That’s 108 000 people.

Medium-sized airports would have about 10 flights per hour, 15 hours per day. Fifteen percent would be in the air at any one time, with an average of 100 people on board. That’s 112 500 people.

Small airports would have about five flights per hour, 12 hours per day. Fifteen percent would be in the air, with an average of 50 people on board. That’s 45 000 people.

Now I can add up all the people. The total equals 400 500 people in the air right now over india.

To sense-check this, let me just divide by 200 (the average number on board a commercial airliner). That’s 2002 planes in the air.

All the numbers are estimated at a higher side but the interviewer looks for the approach.