Delivery time estimation
Design the end-to-end delivery-time estimation shown to a customer at checkout for food delivery: total time = restaurant prep + courier-to-restaurant travel + wait + restaurant-to-customer travel, where the courier isn't even assigned yet. You must produce a single committed delivery window (e.g. '25-35 min') that's trustworthy: under-promising hurts conversion, over-promising erodes trust and triggers refunds. Millions of estimates/day, live traffic, variable kitchen speed. Describe how you compose the multi-leg estimate, how you express uncertainty as a window, and the calibration/business trade-off of the promised time.
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.