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Say you are in a huge hall or a big class and 50 fans are running. You want to switch off one fan. How do you do that?? As of now, we switch off one, wait for some time till decrease in speed is noticeable and if luck is urs, that is the correct fan, otherwise, keep iterating.
Rather, there should be a bank of LEDs on the perimeter of the rim. So we just keep looking at our fan of interest, keep toggling all switches. When LEDs of the fan of interest go blind, done!!