Logic and Estimation Interview Questions (Windows in London and Similar)
Logic and Fermi questions (like “cost to clean every window in London”) test structure, not trivia. Break the problem into assumptions, show the maths, and say what you would verify next.
How much would it cost to clean every window in London? Interviews use questions like this for PMs, engineers, and data analysts. They are not asking for the true number. They are asking whether you can structure uncertainty.
What the Interviewer Wants
- A structure (population → buildings → windows → time → cost)
- Stated assumptions
- Simple arithmetic you can do aloud
- A sense of what you would verify next
- Calm English under a weird prompt
They are not scoring your knowledge of Greater London's window stock.
A Worked Skeleton
- Clarify the ask — all windows, or residential only? Include offices? Inside and outside?
- Choose a funnel — people → households → buildings → windows per building
- Pick round numbers — 9 million people, 2.5 per household, 4 windows per home, plus a commercial fudge factor
- Convert to labour — minutes per window, fully loaded cleaner cost per hour
- Sense-check — is the total in millions or billions? Does that feel like a city budget line?
- List what you would look up — office floor space, building regs, union rates
Say the assumptions out loud. Invite the interviewer to change one number and recompute. That is the senior move — same as <a href="/blog/how-to-answer-interview-questions-like-a-senior">Clarify → Approach → Trade-offs</a>.
Variants You Should Practise
- How many taxis in Berlin?
- How much storage to keep every Slack message for a 2,000-person company?
- How many payment transactions on a Tuesday at a grocery chain?
Engineers: add a systems flavour (throughput, not only money). Data: add a measurement plan. PMs: add a user and a constraint (safety, night work, heritage buildings).
English Under Pressure
Use slow, clear sentences. I will assume... If that is high, the cost still lands in... Next I would check... Career English for this style lives in <a href="/blog/english-for-tech-interviews">English for tech interviews</a>.
Fermi questions reward composure. Practise three of them out loud this week and you will not freeze when London's windows appear.
Frequently asked questions
How do I answer “how many windows in London” style interview questions?
Estimate people or buildings, guess windows per unit, multiply, then cost per window. Say each assumption. Offer a sanity check at the end.
Who gets asked estimation questions?
PMs, some data and analytics loops, and occasionally engineers. The skill is the same: structure uncertainty.
What if my number is wildly wrong?
Wrong plus clear assumptions still passes. Right plus silence often fails. Correct an assumption if the interviewer pushes — that is the test.