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Three Switches, One Visit

Brain Teasers
Logic Puzzles
Easy
Three switches outside a closed room control three incandescent bulbs inside. You may enter the room exactly once. How do you identify which switch controls each bulb?
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Hint 1

A bulb carries more information than on or off.

Hint 2

Incandescent bulbs get hot when they have been on.

Show worked solution

  1. 1. Use a second signal

    Turn switch A on for several minutes so its bulb heats up, then turn it off. Turn switch B on and enter.

  2. 2. Read the bulbs

    The lit bulb is B, the warm unlit bulb is A, and the cold unlit bulb is C.

  3. 3. Key idea

    When one observation is not enough, look for a second measurable property (here, temperature).

Answer: Turn A on for several minutes then off, turn B on, enter: lit = B, warm and off = A, cold and off = C

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