Three switches, three bulbs behind a closed door, and one look inside.

Three switches in a corridor each control one of three bulbs behind a door. You can flip the switches as much as you like but go in only once. The story asks how to match them up. You pick one of four answers. Then you step through the answer and the breakdown.

Why quant interviews test this

This tests counting the outcomes you need, then looking for a signal the puzzle didn't mention. The written solution is at /brain-teasers/three-switches/solution.

The The Three Switches guide covers how scoring works, the strategy that wins, a worked example and the mistakes most players make.

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