Google Maps

How this question breaks down

1. 1TilesCore29 minDraw any part of the world at any zoom in a few hundred milliseconds by cutting the map into a pyramid of small, cacheable vector tiles and styling them on the device.
2. 2RoutingCore27 minFind the fastest route across a continent in milliseconds by searching a road graph that was partitioned and pre-summarised offline, so live traffic changes cost seconds to absorb, not hours.
3. 3ETAsCore25 minTurn millions of phones' anonymous GPS pings into a speed for every road, blend them with how that road usually behaves at that hour, and predict when you will arrive, not just how long the road takes right now.

Work through the parts in order and you’ve covered the full design. Each part is one topic.