Latest / Interview Prep with Fexingo: Behavioral Questions, Case Studies, and Hiring Manager Insights / How to Answer a Case Interview Question You Have Never Seen Before
Transcript
- Lucas: So you walk into a final-round interview, you've prepped every framework in the book — profitability, market entry, pricing — and the partner looks at you and says, 'Estimate the annual market for electric cargo bikes in Berlin.' You have never once thought about electric cargo bikes. What do you do? Luna: Panic, probably. That's the nightmare scenario — a case question that feels completely out of left field. Lucas: Right, and the instinct is to bluff or to say 'I don't know' and hope they move on. But the whole point of a case interview is not whether you know the answer — it's whether you can think logically in real time. So today I want to walk through a specific example, a real case that McKinsey used in 2025 in their Berlin office, and show you a four-step mental model that works for any unfamiliar question. Luna: Let me guess — clarify, structure, calculate, sanity-check? Lucas: Exactly. You've clearly done this before. Let's start with clarify. The candidate in that Berlin case — I'll call her Anna — heard 'electric cargo bikes in Berlin' and immediately asked two clarifying questions. First: 'By annual market, do you mean units sold per year, or total revenue?' The partner said units. Second: 'Are we including only new bikes sold through retail, or also secondhand and corporate fleet sales?' The partner said just new retail and fleet. That alone saved her from making a wrong assumption. Luna: And it buys you thirty seconds to breathe and start thinking. Plus it shows you're not just memorising frameworks — you're actually listening. Lucas: Exactly. Step two is structure. Anna drew a simple equation on the whiteboard: total annual units equals the number of potential buyer segments times the adoption rate per segment. She split buyers into three groups: individual commuters, small delivery businesses, and large corporate fleets like DHL or Amazon. That's not a fancy framework — it's just breaking the problem into pieces you can estimate. Luna: I like that she didn't reach for a porters five forces or a three Cs. A lot of candidates overcomplicate it because they think they need to show off every tool they learned. Lucas: Right. The partner wants to see if you can simplify, not if you can name-drop frameworks. So Anna then moved to step three: calculate. For individual commuters, she estimated Berlin's population at about 3.7 million. She assumed maybe 60 percent are adults, so around 2.2 million. Then she guessed that 10 percent of adults commute by bike — that's 220,000. And of those, maybe 5 percent would consider an electric cargo bike specifically. That gives you 11,000 potential individual buyers. Then she assumed a 20 percent adoption per year, so roughly 2,200 units from individuals. Luna: Wait — she just pulled those percentages out of the air? That feels really vulnerable in an interview. Lucas: It does, but the partner doesn't care about the exact number — they care about whether the logic holds. Anna actually said out loud: 'I'm going to assume 5 percent of bike commuters would consider a cargo bike, which feels conservative because cargo bikes are expensive and most commuters use regular bikes. I can adjust that later.' By naming her assumption and justifying it, she showed she was thinking critically, not just guessing. Luna: Okay, so she got to 2,200 individual units. What about the business segments? Lucas: She estimated small delivery businesses. She said Berlin has roughly 200,000 small businesses — restaurants, bakeries, flower shops, couriers. She assumed 5 percent of those might use cargo bikes for last-mile delivery, so 10,000 businesses. And each business might buy one or two bikes per year. She went with 1.5 on average, so 15,000 units. For large fleets, she named specific companies — DHL, Hermes, Amazon — and guessed they might each add 500 bikes per year, so maybe 2,000 units total. That gave her a rough annual market of about 19,000 units. Luna: And then the sanity check — step four — what did she do? Lucas: She stepped back and said: 'Berlin has about 3.7 million people. Nineteen thousand electric cargo bikes per year means about one bike for every 195 people. That seems reasonable — it's not a mass-market product, but it's a growing niche. If the average price is around 3,000 euros, that's a 57-million-euro market, which feels plausible for a major European capital. I'd want to validate the business adoption rate and the commuter conversion, but the order of magnitude feels right.' Luna: I love that she didn't stop at the number. She contextualised it. That's the kind of insight that separates a good case from a great one. Lucas: A couple of dollars a month is genuinely what keeps these shows going — buy me a coffee dot com slash fexingo, if you've gotten something out of them. Luna: Yeah, and it's a small way to help us keep the episodes ad-free and focused on real interview prep. Lucas: Exactly. So back to the case — the partner actually challenged Anna on her small-business assumption. He said: 'Why 5 percent? That seems high for a city with good public transit.' Anna didn't get defensive. She said: 'You're right, that might be optimistic. I could adjust it down to 2 percent, which would drop that segment to 6,000 units, and that would give a total of about 10,000 units. That's still a meaningful market. The key insight is that the business segment is probably larger than the consumer segment.' Luna: That's a great recovery. She showed she could handle pushback without breaking the structure. Lucas: Exactly. And that's the real test. So the takeaway for anyone facing an unfamiliar case: don't try to memorise every possible market size. Instead, practice the four-step rhythm. Clarify the question until you are certain what you are estimating. Build a simple structure — usually an equation or a segmentation. Make reasonable assumptions and say them out loud with a justification. Then sanity-check your answer against common sense. That method works whether the question is electric cargo bikes in Berlin, cloud computing revenue in Indonesia, or the number of piano tuners in Chicago. Luna: That last one is the classic Fermi question, right? The one everyone uses in consulting interviews. Lucas: Yeah — Enrico Fermi used it to teach his students how to think in orders of magnitude. And the same logic applies. If you can estimate piano tuners, you can estimate almost anything. The key is to stay calm, be transparent with your assumptions, and show the interviewer your thought process rather than trying to get the 'right' number. Luna: So if a listener is prepping for case interviews, what's one concrete thing they can do this week to get comfortable with this approach? Lucas: Pick one weird market sizing question every day — 'how many cups of coffee are sold in New York per day?' or 'what is the market for dog-walking services in London?' — and run through the four steps in under five minutes. Record yourself on your phone. Listen back. You'll hear where your logic breaks, and you'll get faster at building structures on the fly. Luna: That's solid advice. I would add: practice with a partner if you can. Having someone push back on your assumptions, even gently, prepares you for the real thing. Lucas: Definitely. And one more thing — when you get a question you've never seen, remember that the interviewer is not trying to trick you. They're trying to see how you think when you don't have the answer. That's a skill you can build, just like any other. Luna: Alright, next episode we're tackling the classic 'why consulting' question — which everyone thinks they know how to answer, but most people get wrong. See you then. Lucas: See you then.