Estimate number of people in an Airport: PM Interview | Guesstimate.

Guesstimate is a popular type of question asked by companies like Google, Meta, Microsoft in their PM interviews.

In the fast-paced realm of product management, the ability to make quick, data-driven decisions is crucial.

This skill is often tested through ‘guesstimates’ in interviews, a unique blend of guesswork and estimation that challenges even the most seasoned professionals.

As a product manager with years of experience, I’ve faced and posed these intriguing questions, understanding their significance in assessing analytical prowess.

Guesstimates are not just about crunching numbers; they’re a test of logic, creativity, and problem-solving skills under ambiguous conditions.

In the real world, product managers frequently encounter situations with incomplete data. How you navigate these uncertainties during an interview can be a strong indicator of your potential on the job.

Guesstimate questions in the product interviews were popularised by Google. Google started using these types of questions to gauge their candidates thinking capability.

You might have heard about questions like how many tennis balls can fit an Airplane, How many burgers sold in McDonalds etc being asked in Google interviews.

We have created a Guesstimate cheatsheet which will help PMs with the numbers and estimations. If you wish to download it, Comment “Guesstimate Cheatsheet” and will share the link to download. Free for limited time!

The Four-Step Guesstimate Approach:

1. Clarification of the Question:

• Objective: Begin by ensuring a mutual understanding of the question with the interviewer. It’s crucial to know precisely what’s being asked before diving into problem-solving.
• Example: If the question is “Estimate the number of passengers present in an airport,” confirm whether it includes staff, transit passengers, etc.

2. Asking Clarifying Questions:

• Purpose: This step involves making sure your assumptions align with the interviewer’s expectations.
• Example Questions:

“Should I consider the time of day or season?”

“Are we focusing on a specific airport or a general average?”

3. Choosing the Approach:

Top-Down vs Bottom-Up: Select an approach based on which is more suitable for the problem.

• Top-Down: Start with a larger known quantity and narrow it down. For example, start with the total daily passenger traffic and narrow it down to a specific time.
• Bottom-Up: Build up from smaller, detailed data points. For instance, consider the number of flights, average passengers per flight, etc.

4. Summarising Findings:

• Conclude with a Summary: Share your final estimate, backing it up with the logical steps you took.
• Gut Check: Validate your answer against common sense and industry benchmarks where possible. Does the number seem realistic?

Now that we have had a basic understanding of guesstimates, here’s a candidates first-hand encounter with one of the Guesstimate questions in the PM interview.

Estimate number of people in an Airport.

Me: Just reiterating, the goal is to find the number of people present in an airport on a particular day.

Interviewer: Yes

Me: Is it any particular airport or just an average number of people in any airport?

Interviewer: Since you are from Mumbai, let’s consider the Mumbai airport.

Me: Along with the passengers are we also considering the airline support staff, security, ground crew, and pilots as well?

Interviewer: For simplicity purposes let’s just consider Passengers.

Me: Are we also considering Military flights and private jets as well?

Interviewer: No, just passengers using commercial flights.

Me: Mumbai has 2 Airports Domestic and International and total of 2 runway shared between both of them. Should we consider both airport?

Interviewer: Let’s consider the International airport.

Me: Are we considering any particular day? Or the holiday season?

Interviewer: Let’s consider it for today (Friday), but let’s keep it simple, and let’s not consider it be a holiday season.

Me: OK, I would like to approach it in a Bottom-up way, Here in our case we have the Mumbai Airport.

Mumbai airports 2 runways cross each other, so at a time only one runway is in use.

We have arrival and departure, so we will consider passengers who arrive as well as depart from Mumbai Airport.

And Mumbai international airport is used by both domestic and international flight.

Am I good till now?

Interviewer: Yes, please continue.

Me: So we will divide our passengers who depart and arrive into Domestic and International. It’s important to make this distinction,

I will explain why in a bit.

Now I want to check how many flights can use the runway in any given hour. In the best-case scenario.

In the peak hour, I believe it takes 1.5–2 min for a flight to take off or land, in the 2 minutes, no other flights are on the runway.

Since we are running with one runway.

I would like it to be considered to be 2 min for easy calculation. Is my assumption fair?

Interviewer: So you are saying that the maximum capacity is 1 flight every 2 minutes

Me: Yes

Interviewer: Yes, I believe that’s a fair assumption.

Me: So now that we have arrived at the number of 2 minutes/flight for the peak frequency time we need to assume flight time for Moderate frequency and low frequency period as well.

For moderate frequency, I believe the flights to be every 4 minutes, and for low frequency, it to be every 6 minutes.

Is this assumption fair?

Interviewer: yes

Me: So let’s calculate the flights during peak frequency.

It’s 60/2= 30 flights in an hour.

For moderate frequency: 60/4= 15 flights/hour

And for low frequency: 60/6= 10 flights/hour

Now we need to define when is it peak, moderate, and low frequencies in the Mumbai Airport

I believe the time can be divided as Peak: 8 hours, Moderate: 10 hours, and low: 6 hours.

Is my assumption fair here?

Interviewer: why are we dividing it as 8, 10 and 6? And not any other number?

Me: Based on my assumption domestic flight traffic can be shown as

4 PM -12 PM Peak frequency

6 AM — 4 PM moderate frequency

12 AM — 6 AM Low frequency

I believe since it’s a Friday, Many will be flying between 4–12 PM, heading back home after office and some heading for a weekend getaway.

While 6 AM — 4 PM is the usual time this might be moderately booked.

And the time of 12 AM -6 AM many domestic travelers might not choose to travel since they will not be able to get any transport back home or to the hotel when they land.

Similarly for international flights

10 PM — 6 AM: Peak hours (since most international flights depart at this time)

6 AM — 4 PM moderate frequency

4 PM — 10 PM low frequency.

So based on these times and assumptions, I was able to get the numbers 8, 10, and 6. Is it fair?

Interviewer: Fair assumption, let’s continue.

Me: So based on the Assumption we made, lets calculate the number of flights in a particular day for domestic.

Domestic Flights:

Peak hours: 8 * 30 = 240

Moderate hours: 10 * 15 = 150

Low frequency: 6 * 10= 60

Total domestic flights in a day = 240+150+60

= 450 flights/day.

Similarly for International flights:

= 450 Flights

Am I good till now?

Interviewer: Yes, looks good to me.

Me: Now based on the number of flights we will derive the number of passengers on the domestic and International routes.

For domestic flights the most commonly used aircrafts are A320 and Boeings aircarfts with max capacity around 180. Their are also flight with much smaller capacity like 60 to 100 but they are comparatively lesser from Mumbai.

Since Mumbai is one of major airports in Mumbai. So based on that, I would like to consider the flights with max capacity of 180.

Is this a fair assumption?

Interviewer: Yes

Me: Ok, now based on the capacity I will derive the number of passengers flying in the 3 assumed traffic periods.

In India people still prefer to travel via Train or Bus, So there is a less chance that all the flights are 100 % booked.

So I would like to consider during the peak traffic the flight is 80% booked and during the moderate and low traffic period it is 60–70% booked.

Lets go with 70% since the number of flights are also less in these periods, so that should compensate for the same.

Is my assumption fair?

Interviewer: Looks good to me.

Me: Ok now based on these assumption lets calculate number of passengers during all the traffic periods.

Peak= 240 * (180*80/100)

= 240 * 144

= 34,560

Moderate= 150 * (180*70/100)

= 150 * 126

= 18,900

Low= 60 * 126

= 7,560

So thats a total of 34,560+18,900+7560

=61,020

Now for the international flights we have a little different scenarios.

Airlines use 2–3 types of flights but one of them is A380, which as we all know is the Airbus famous double decker airplane.

It has a capacity of 500, based on the airline.

And the other airplanes are A350, Boeing 777 and 737. These have a capacity of 250–300. I would like to consider them to be around 300.

Is my assumption fair?

Interviewer: How are we going to calculate of A380, whats the purpose of considering it separately?

Me: A380s are used by airlines during the highest traffic time and in the highest traffic routes.

So based on my assumption they will be used only during the peak hours.

And the number of A380s are less so they will be only 10% or less of all the flights during that period.

So I will calculate them separately for the peak hours.

Is my assumption fair?

Interviewer: OK that sounds interesting, continue.

Me: OK now lets calculate the the passengers for the same.

Again I will consider the same 80% occupancy during the peak and 70% during the other time.

I will consider the A380 to be little less than 10% and round it off to 20.

Peak:

= A380 + Other Airplanes

= 20 * (500 * 80/100) + 204 * (300 * 80/100 )

= (20 * 400) + (204 * 240)

= 8000 + 48,960

=56,960

Moderate:

= 150 * (300*70/100)

= 150 * 210

= 31,500

Low Traffic:

= 60 * (300*70/100)

= 60 * 210

= 12,600

So the total people travelling via international flights comes to around

= 56,960 + 31,500 + 12,600

= 101,060.

Does my count look fair to you.

Interviewer: So your International travellers are more than domestic?

Me: Yes, as I mentioned earlier, Indians still prefer traveling via Train or bus domestically.

And for travellers travelling international, plane is the best choice and provided Mumbai is one of the most important airport, most international flights in India will fly from Mumbai.

Interviewer: Sounds cool, do we have a final number?

Me: Yes , if I add the domestic and international passenger we should be able to come to the total passenger in Mumbai airport in a day.

=101,060 + 61,020

= 162,080.

So that would be total passenger in a single day in Mumbai Airport.

Interviewer: good that sounds close to the actual number.

Summary:

The candidate does a nice job of listing down all the case and provides a detail breakdown of all the scenarios. This shows the candidates ability to use the first principal.

The candidate also constantly was asking the interviewer if the assumption was right, this helped them to be on the same page.

Guesstimates like estimating the number of passengers in an airport require a structured approach, clear communication, and logical reasoning.

It’s not just about the final number but how you arrive at it.

This methodology is not just useful for interviews but also in practical product management scenarios, where making informed estimates is part of everyday decision-making.

Mastering guesstimates is a journey of enhancing your analytical skills and business acumen.

Practice is key.

Challenge yourself with diverse scenarios, learn from each attempt, and remember, the process is as important as the answer.