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5 Wood-cutting Machines

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


Puzzle ID:#18843
Fun:*** (2.29)
Difficulty:*** (2.83)
Submitted By:nenad**
Corrected By:Nytwynd




In a wood-cutting factory, five 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 five 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 five 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 five switches outside the room to the five 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?


1) Turn off two machines while inside. Let's call those two machines turned off with inside switch 'inactive', and call remaining three machines 'active'.

2) Go outside.

3) Find out which two outside switches correspond to inactive machines, and which correspond to active machines. To do that, try all of following ten combinations, where 'x' represents turned-off outside switch, while 'O' represent turned on switch: xxx00,xx0x0,x0xx0,0xxx0,xx00x,x0x0x,0xx0x,x00xx,0x0xx,00xxx . Only one of these combinations is a 'silent' combination that eliminates all sound, by turning off all three active machines.

4) Label outside switches related to active machines (those marked with 'x' in the silent combination) with A, B, and C. Label outside switches related to inactive machines with D and E.

5) For inactive machines, turn D off and E on.

6) For active machines, turn A and B on, and turn C off.

7) Wait enough time for machine connected to C switch to cool off, and then turn B off, too.

8) Go inside.

9) The only machine that is still working inside is connected to outside switch A.

10) Check both machines that were active and are now turned off. The one that is warmer is connected to switch B (since it was on most recently), and the cooler one is connected to switch C (which was turned off first).

11) Turn inside switches on for both 'inactive' machines. The one that starts working is connected to outside switch E. The one that is still not working is connected to outside switch D.


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