BLACKBOX

Puzzles

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There is a battery powered fan inside the black box. Using only one among 3 outside switches – A, B and C, you can on and off the inside fan but you don’t know which one is that? You can on-off any switch as many times as you want but can open the black box only once. After opening the black box you can’t touch the fan or the switch. Here we are supposing the fan rotation will be stopped immediately after switching off the connected switch. The black box is also soundproof meaning you can’t hear the noise of rotating fan outside the black box.

How do you know with 100% surety which switch is connected to the fan? What should be your strategy?

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Solution

Switch on ‘A’ for 10 minutes and then switch off ‘A’.

Now switch on ‘B’ and open the blackbox.

If the fan is rotating, then it means a fan is connected with switch ‘B’.

If the fan is not rotating then check the battery. If the battery is hot then the fan is connected with switch ‘A’ else the fan is connected with switch ‘C’.