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Google·Software Engineer·Technical Phone Screen·Intermediate

IntermediatePrefer not to say
May 2026Bay Area

Summary

Google phone screen, got hit with a classic estimation question about air traffic over the Bay Area. Short interview, nothing too wild, but I definitely fumbled my structure a bit.

Questions Asked (1)

Q1

How many airplanes are currently in the air over the Bay Area?

Adaptability & AmbiguityProduct Analytics & Metrics
Author's notes

Fermi estimation.

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AI HintsAI Generated

Suggested Approach

Clarify the scope (geographic boundaries, time frame, and whether 'Bay Area' includes all airports or just major ones) and state your assumptions. Then use a structured estimation approach, breaking the problem into components like flight frequency, airspace capacity, and average flight duration, and calculate a reasonable range.

Pro tip: Acknowledge that this is a Fermi problem where the process matters more than the exact number; demonstrate your ability to make reasonable assumptions and sanity-check your answer.

1. Clarify the question

Ask clarifying questions to define the Bay Area boundaries, time frame (e.g., current moment, average day), and whether to include all aircraft types (commercial, private, cargo).

2. Identify major airports and flight volumes

List major airports in the Bay Area (SFO, OAK, SJC) and estimate their daily flight operations (takeoffs and landings) based on known statistics or reasonable guesses.

3. Estimate average flight duration in the airspace

Determine the average time an aircraft spends within the defined Bay Area airspace, considering approach and departure paths (e.g., 10-15 minutes).

4. Calculate the number of planes in the air

Use Little's Law: number of planes = arrival rate × average time in system. Convert daily operations to per-minute rate and multiply by average duration.

5. Sanity-check and refine

Compare your estimate to known data (e.g., FlightRadar24) or adjust for factors like peak hours, general aviation, and overflights, and present a range.

Key Points to Mention

  • Clarify assumptions: geographic boundaries, time frame, aircraft types.
  • Use Little's Law (L = λW) to relate arrival rate and time in system.
  • Estimate daily operations at SFO, OAK, SJC (e.g., SFO ~1,200 operations/day).
  • Average time in airspace: ~10-15 minutes for arrivals/departures.
  • Include general aviation and overflights if relevant.
  • Sanity-check with real-time flight tracking data or known statistics.

AI-generated suggestions, not part of the candidate's original notes. May be inaccurate — verify before relying on them.