Round 5 · Final Interview
Market Sizing & Brainteasers – Fermi Estimation
Fermi estimation formulas and market sizing frameworks — practise logical thinking and communicating clearly under pressure.
Market Sizing – Top-Down vs Bottom-Up
Top-Down Approach
Start from: Total population → relevant segment → penetration rate → frequency → spend per person
Example: "Size of bubble tea market in Hanoi"
Population: 8M → Urban young adults (18-35): ~2.5M → % who drink bubble tea weekly: ~40% = 1M → 2 cups/week × 50,000 VND = 100K VND/week/person → ~100B VND/week = ~5.2T VND/year
Population: 8M → Urban young adults (18-35): ~2.5M → % who drink bubble tea weekly: ~40% = 1M → 2 cups/week × 50,000 VND = 100K VND/week/person → ~100B VND/week = ~5.2T VND/year
Bottom-Up Approach
Start from: Number of supply units (stores, outlets, providers) → average output per unit → aggregate
Example: "Size of bubble tea market in Hanoi"
~2,000 bubble tea stores in Hanoi → average 150 cups/day × 45,000 VND → 6.75M VND/store/day → 6.75M × 2,000 × 365 = ~4.9T VND/year
~2,000 bubble tea stores in Hanoi → average 150 cups/day × 45,000 VND → 6.75M VND/store/day → 6.75M × 2,000 × 365 = ~4.9T VND/year
Tip: Always state your approach first ("I'll use a top-down approach starting with total population"), round your numbers intentionally ("I'll assume ~8 million for Hanoi for simplicity"), and sanity-check your final answer ("Does ~5 trillion seem reasonable for a city this size? Let me cross-check with the bottom-up...").
8 Practice Market Sizing Questions
1
How many cups of coffee are sold in Vietnam per day?
Approach hint: Population 100M → coffee drinkers % → cups/day/drinker
2
Size of the ride-hailing market in Ho Chi Minh City (annual revenue)?
Approach hint: Population → commuters → ride-hailing users % → trips/week → average fare
3
How many smartphones are sold in Vietnam per year?
Approach hint: Population → eligible buyers → replacement cycle → first-time buyers
4
What is the annual revenue of Vietnamese e-commerce market?
Approach hint: Internet users → online shoppers % → orders/year → average order value
5
How many piano lessons are given per year in Hanoi?
Approach hint: Population → school-age children → parents who enroll → lessons/week
6
Estimate the size of the tutoring market in Vietnam
Approach hint: School-age population → students getting tutored % → hours/week × rate
7
How many taxis are there in Ho Chi Minh City?
Approach hint: Population → commuters needing taxis → trips/taxi/day → fleet size
8
What is the annual market for skincare products among Vietnamese women?
Approach hint: Female population → age 18-55 → % using skincare → spend/month
Classic Brainteasers & Logic Puzzles
Why are manholes round?
A round cover cannot fall through a circular opening of the same size—regardless of rotation. Other shapes could be dropped through diagonally. Also lighter & easier to roll/move.
How would you move Mount Fuji?
No right answer—shows creative thinking: reframe the question (move the people around it? rename a hill?), escalate to absurd (remove atom by atom), or redefine constraint (what counts as "moving"?).
You have 8 balls, one slightly heavier. Min weighings to find it?
Minimum 2 weighings: split into 3-3-2. First weigh group of 3 vs 3. If balanced → heavier is in 2 → one more weigh. If unbalanced → take 3 heavier → weigh 1 vs 1 → if balanced, 3rd is it.
How would you design an airplane seat for tall people?
Understand root cause: limited leg room. Solutions: stagger seats, offer extra leg-row at premium, flexible seat-back mechanism, configurable cabin for tall passengers, or redesign boarding to seat tall passengers by exits.