Beach Day by Rodger Bliss
April 2024
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A Difficult Lighting Issue

bulbThere are three light bulbs on the second floor of a building and there are three switches on the ground floor which are used to turn on the light bulbs (one switch turns on one bulb).

Joshua is on the ground floor and he knows that the three light bulbs on the second floor are all turned off. How can Joshua just go up to the second floor ONCE to determine which switches control which light bulbs ??

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2 guesses to A Difficult Lighting Issue


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  • brent

    Joshua can turn on two switches and wait 10 minutes, then turn off one of them.
    Go to the 2nd floor. The light that is still on is the switch that was on, the light that is off but still warm is the switch he had turned on for 10 minutes and then turned off, the cold bulb is the one he never turned on. ? Might not work with LED’s…

  • Brent is absolutely right. This riddle will make no sense in the future. You are today’s winner.

    First, turn on one of the switch for a long time, e.g. 20 minutes, then switch it off, then turn on another switch, then immediately go to the second floor. On the second floor, see which light bulb is turned on. that light bulb corresponds to the switch which is turned on at last. Sense the two light bulb which is not turned on, see which one is hotter, the one which is hotter corresponds to the switch which has been turned on for a long time but turned off subsequently.