Three Switches, One Bulb
Three switches downstairs control one incandescent bulb upstairs; all switches are initially off, and you cannot see the bulb from the switches. You may manipulate the switches however you like. What is the minimum number of trips upstairs needed to determine with certainty which switch controls the bulb?
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Source: Classic switches-and-bulb deduction puzzle in wide circulation; no traceable original source. Statement written for AxiomIQ.