How many passengers is at the airport at noon?

In general, when these kinds of questions are asked, it would be great to see your full reasoning, not only what you said to the interviewer and what he/she replied. I also doubt that it is true that an interviewer just stopped the case after your answer, even though it is a wrong answer (or at least an answer that is bringing you nowhere).
It is also unclear what you mean by inflow or outflow. In one minute, hour, or what? 

That said, let me do the exercise for you by giving you some hints. Consider that in the airport you have 

• people who just landed and have to leave the airport. For some of them, you have some people waiting to grab them
• people who just landed and are going to take another flight
• people who have to start their travel from the airport
• people who work at the airport.

For each leaving or arriving you can estimate the number of people inside (let’s say 180 people just for the sake of it).

Arrivals
Consider that some people arriving, will remain inside the facilities to take another flight in maybe 2 hours on average (small complication).

The others who just landed and are leaving will stay in the facility for 30 mins on average (?), so you calculate how many flights landed in the previous 30 mins.
Consider that some people are attending to grab passengers at the airport.

Departures
Then you have people in the queue for departure (including the one who landed and take another flight from the same airport), that will stay on average inside the facility for 2 hours (?). How many flights are departing over the next two hours? It depends on the number of tracks and the busyness of the airport, but it is easy to estimate. So, just compute “number of flights departing” times “the average number of people per flight”.

People working at the airport
Estimate the type of shops and number and the people needed to run the operations of the airport, quite easy.

I think the reasoning now is clear.

Hi there,

Respectfully, I have no idea why you decided to bring up irrelevant information when he already gave the answer! He gave you flow. You cannot add a 2nd flow.

Think about it this way: If you want to measure how much oil is in a pipeline, would you agree that you can look at 1 spot in the pipeline and count how many units flow past you? What you did in the above case was count in two spots!

Here, you have a few thoughts:

1) There's no “solving” this. He gave you the answer. It's 6,000. There's nothing more to it

2) If you wanted to market size this you could:

a) Take a bottom-up approach and visualize the airport. You could think about # of terminals OR # of flights taking off at a given time, OR the flight board, etc. etc.

b) Take a top-down approach, but this would generally be a quite difficult, and “guessing” way to do it.

3) There's a much larger problem here that has nothing to do with market sizing. It has to do with your critical thinking and case solving. You need to put in some serious work to better learn/understand how cases work!

Sorry to be so direct, but if you put in the hard work you can likely get there :)