Live order tracking and ETA
Design the live order-tracking and arrival-prediction service shown to a customer after they place a delivery order: a moving courier marker, a continuously-updated 'arriving in N min' estimate, and a notification when the courier is ~2 minutes away. 2M active tracked orders at peak. Cover how the estimate is computed and refreshed, and how the 'almost there' trigger works without false alarms.
What a strong answer looks like
Clarify scale and constraints first. Propose a clean component breakdown, then go deep on the hard parts (data model, bottlenecks, consistency, failure modes) and name the trade-offs you are making.
Clarify4:30 left
Estimate4:30 planned
Design13:30 planned
Deep dive10:30 planned
Failure7:00 planned
Which questions mattered is sealed until you submit. Telling you now would just be handing over the edge cases.
Run or narrate your approach, then ask the coach.