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Sunday Puzzle-45
User Rating: / 1
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Written by Administrator   
Sunday, 14 July 2013
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Puzzle courtesy of Barry R. Clarke, columnist for The Daily Telegraph and international puzzle expert

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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?

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Comments (1)
1. Written by frail on 05:11 15 2013 .
 
 
I assume that the machines can be heard from outside 
where the backup switches are. If so: 
 
Lets label the machines as ABCD. 
The brother turns of A&B before going outside. 
Outside, he flips switches in pairs till he 
hears complete silence. (i.e. none of the machines 
run). At this point, outside there will two 
switches that are off and two that are ON. 
The ones that are OFF correspond to machines 
C & D. So lets label them CD1 and CD2. Similarly 
the other two are labeled AB1 and AB2. Now 
the brother sets CD1 and AB1 two ON and turns 
off CD2 and AB2. He then walks back in. 
He will see one machine running. If C is 
running then CD1 = C & CD2 = D. If D is 
running then CD1 = D & CD2 = C. Thus 
he determines which switch is for C and 
which switch is for D. Now for A&B, he turns 
on machine A's switch inside. If machine A 
runs, then AB1=A & AB2=B. If not, AB1=B 
& AB2 = A.
  

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