Classic estimation question and I still fumbled it.
Break the problem into two parts: first estimate the number of airports in the US using a top-down approach based on population and travel demand, then extrapolate to worldwide using similar logic or known ratios. Clearly state your assumptions, walk through the math, and sanity-check your final numbers against known facts.
Pro tip: Acknowledge that the exact number isn't the point—interviewers care about your structured thinking and ability to make reasonable assumptions. Mention that the FAA reports about 5,000 public-use airports in the US, but if you don't know, derive it logically.
Ask whether 'airports' includes all types (public, private, military) or just commercial airports. Also clarify if you should include heliports and seaplane bases.
Start with population (≈330M) and assume one airport per certain number of people based on travel demand. Alternatively, use known data: ~5,000 public airports, ~14,000 private, totaling ~20,000.
Scale from US using population ratio (world ≈ 8B, US ≈ 330M, so ~24x) but adjust for lower air travel penetration in developing countries. Alternatively, use known global estimate of ~40,000-50,000 airports.
Compare your estimates to known figures (e.g., CIA World Factbook lists ~41,000 airports worldwide). Acknowledge uncertainty and state that the exact number is less important than the method.
AI-generated suggestions, not part of the candidate's original notes. May be inaccurate — verify before relying on them.