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## Wood-cutting Machines

Logic puzzles require you to think. You will have to be logical in your reasoning.

 Puzzle ID: #4820 Fun: (2.51) Difficulty: (2.72) Category: Logic Submitted By: arij

In a wood-cutting factory, four large sawing machines stand in a windowless room. Each machine has an on/off switch attached, there being no doubt as to which switch controls which machine. Outside the door to the room are four back-up on/off switches, one for each machine inside. The power for each machine must first pass through the back-up switch, and then the machine switch before reaching the saw. The problem is, the new manager cannot decide how these back-up switches match with the machines inside the room. One day, the manager's brother visits. The manager takes him inside the sawing room where all four machines are at work and explains the problem. The brother announces that he intends to leave the room and that when he returns he will be able to match correctly the four switches outside the room to the four machines inside. The brother works alone, cannot see the machines from outside the room and solves the problem purely by operating switches. How is it possible?

Turn two machine switches off, go outside the room and, by listening, test all six combinations of two back-up switches to find which two match the two "machine-on" switches inside the room. Only one of the six combinations of two back-ups gives complete silence when switched off and these "machine-on" back-ups match the two "on" in the room. Now, before returning to the room, leave one "on" off and one "off" on leaving the other "on" on and "off" off, that is, only two switches are activated. Back inside the room, we can now match the switches by seeing which "on" machine is now off and which "off" is now on (when we activate its switch).
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