Sizing a Shopping Mall's Passenger Elevator Fleet for Peak Weekend Traffic
Company: Google
Role: Product Manager
Category: Product / Decision Making
Difficulty: medium
Interview Round: Other
You are asked to estimate how many passenger elevators a shopping mall should have. The interviewer gives no details about the mall, so you choose its size, layout and traffic yourself, state each choice as an assumption, and arrive at a number you can defend.
```hint Size it for the rush
Which stretch of time should the elevators be designed around, and what really limits how many shoppers a single car can carry during it?
```
### Clarifying Questions
- How big is the mall: how many shopping levels, and are there parking levels above or below that shoppers reach by elevator?
- Is it a city-center mall that people reach on foot or by transit, or a suburban mall where most visitors arrive by car?
- Should the count cover only passenger elevators, or also the service and freight elevators that stores use for deliveries?
- Does the mall have escalators and stairs between shopping levels, and should they be assumed to carry part of the traffic?
### What a Strong Answer Covers
- A clear equation that separates demand (elevator trips in the busiest window) from supply (riders one car can move in that window).
- Labeled, round-number assumptions for visitors, peak timing, trips per visit and the share of trips that use elevators rather than escalators.
- A round-trip time for one car built from building height, car speed, stops and boarding time, using a realistic load instead of the rated maximum.
- Practical floors on the number: step-free access to every level, banks spread across the building, and spare capacity for maintenance.
- A sanity check of the final count and a note on which assumption moves it most.
### Follow-up Questions
- The mall adds a multiplex cinema on its top floor. How does your estimate change, and when would the busiest moments now occur?
- Once the mall is open, what data would you collect to check whether the elevator count is right?
- The budget covers only half the elevators you estimated. What would you do to keep waiting times acceptable?
Overview: An estimation question that asks how many passenger elevators a multi-level shopping mall needs. It tests setting up a demand-versus-capacity equation, choosing labeled assumptions about peak traffic and escalator use, modeling one car's round trip and realistic load, and sanity-checking the final count.