The Bulb Man
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I have to admit that I didn't come up with this. Michael Coles sent it, but I liked it, so here it is:
I have an old school riddle for you.
There are eight rooms, each with a single door to go in and out. Four of the rooms have light switches and four of them have light bulbs. You can enter and exit each room exactly one time. After you leave a room, the door closes and locks behind you. Your job is to determine which light switches control which light bulbs.
The rooms look like this:
What do you do?
Steve Jones
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Steve
Just want to confirm is the answer logical or something stupid like
you switch on light switch for "room one" wait for 10 minutes , go to 2nd room switch on light switch for "room two" wait for 10 minute and then switch off. go to third room switch on light switch.
Now go to the rooms which have bulb
if the bulb is ON and HOT then it is controlled by Room 1
if the bulb is OFF and HOT then it is controlled by Room 2
if the bulb is ON and NOT HOT then it is controlled by Room 3
if the bulb is OFF and NOT HOT then it is controlled by Room 4
Kindest Regards,Amit Lohia
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Why is that not logical? Light bulbs exhibit characteristic behaviors, one of which is giving off light, and one is giving off heat. Any property of a lit bulb should be fair game in the solution. It's 'off the beam' thinking, rather than 'out of the box'.
And then again, I might be wrong ...
David Webb -
I wish someone have an answer better than mine (looking for 'out of the box'
)
Kindest Regards,Amit Lohia
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You'd be surprised. This was an actual question I got on a job interview... they used it to try to gauge your thought processes. I gave a pretty aggressive answer: grab the switches and yank the bulbs through the walls. Obviously they wouldn't be any use to anyone at that point, but you'd know which switch was connected to which bulb
BTW, your answer is completely logical. If you number each bulb from 1 to 4 and then convert those numbers to binary:
00, 01, 10, 11
The high bit could represent on/off (1 = on, 0 = off). The low bit could represent hot/cold (1 = hot, 0 = cold). Using your method you have assigned two separate binary attributes to each bulb. That gives you exactly four different pairs (2 ^ 2), which is exactly what you need to tell them apart.
Most people get lost looking for that second binary attribute (everyone knows on/off). I think the math behind the logic is more interesting than the riddle itself.
OK, here's a couple more oldies but goodies so you don't feel cheated. The first one, at least, will require you to think "out of the box":
You're in windowless room with 12 inch thick solid steel walls and the door is welded shut. You have a small table and a hacksaw. How do you get out?
You're in a pit with Saddam Hussein, a deadly cobra and a lawyer. You have a gun with two bullets. What do you do?
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Hmmmmmm...
You didn't mention the ceiling composition. How about, climb onto the table, saw through the ceiling and climb out?
Stand behind the cold-blooded lawyer until the cobra bit Sadam Hussein (since snakes tend to strike at heat), shoot the snake and the attorney, and climb out of the pit.
In a previous life, I worked in an oil company warehouse where we canned oil and shipped it around the country. Every Friday we had to clean the floors, so we'd dump industrial solvent all over, get our sqweegie and go over the whole warehouse. We'd take turns bringing in a selection of these kinds of brain twisters to keep the brains going while the body went on auto-pilot. You haven't lived until you try to solve a set of these puppies while the fumes of chemicals strong enough to disolve hgh density lubricant waft up from the floor. I still love these. Thanks for brightening up an otherwise drab Friday!
And then again, I might be wrong ...
David Webb -
I thought the deadly snake and the lawyer were the same, so only two bullets would be needed.
Either that, or this could be one of those scenarios where you take advantage of the fact that, due to professional courtesy, the snake won't bite the lawyer.
<Recloaking @When="Now />
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Sorry to be boring guys - but have we not strayed just a tad from the original question?
I'd like to know if we can assume that the initial state of each switch is OFF.
If not, then the task is a bit harder!
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Um...Would holding the door open work.
So, open each door in turn an note the state of the bulb.
Press switch 1. Check each bulb in turn until you find the one that has changed state.
Press switch 2. Check the other bulbs until you find the one that has changed state.
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Simplistic, brute force, wouldn't scale - I know. But, for four doors it might work.
There is no problem so great that it can not be solved by caffeine and chocolate. -
You're in windowless room with 12 inch thick solid steel walls and the door is welded shut. You have a small table and a hacksaw. How do you get out?
How about use the hacksaw to chop the table into two halves.
We all know 2 halves make a (w)hole
walk out of the hole
To take it further.
You can then shout and shout until you get ho(a)rse
Get on the horse and ride away
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How long does it take for a light bulb to get hot? By the time you have mucked around with the light switches, then feeling each bulb in each room you will end up with two bulbs that are on and are hot.
Is this another example of how theory and practice can be two very different things?
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Hmm... the ON/OFF/HOT/COLD test is a logical solution and it probably is the correct answer. But if I you check the question word by word, some interesting possibilities can be found (other than breaking walls :-)):
- "After you leave a room, the door closes and locks behind you" - this could mean, that at the beginning all doors are open... or at least you can open any door and it could stay open if you don't enter the room
- material of the doors is not mentioned - i.e. doors can be made of glass or have a window/peekhole
So, if asked in an interview, I would probably point out that the situation is not well defined... especially when there is nothing about the initial state of the bulbs!
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We have another solution:
Turn one light on.
Go to the next light switch and turn it on and off repeatedly.
Go to the next light switch, unscrew it and then pull really hard on the electric cable until it moves.
Leave the last switch off.
Then go to the rooms and ta da! The answer:
There will be one light on, one light pulled up tight against the ceiling, one light burnt out and one light off.
The other plans we have had involve not going into the rooms as they only lock when you leave, so just open the door and look at the lights bulbs, or block the doors from closing with items of clothing.
The last one involved finding out who set up this dumb building in the first place and asking them. Possibly with a baseball bat.
Do I get a prize for the most annoying answer?
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Some people obviously never wired up a light! Even this woman knows that the cables don't go direct from switch to bulbholder - there's inevitably a junction box between. So the destructive solutions involving yanking cables just would not work!!
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This is going to be a REALLY long interview if you wait for one of the bulbs to burn out, given that it could have 1200 or so hours of life available. Hopefully the interviewer has some other activities planned.
And even if the wiring did go directly from the switch to the light fixture, I've been in enough walls to know how well anchored the wiring is. Pulling the wire only works in a 3 Stooges episode.
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Buy the ticket, take the ride. -- Hunter S. Thompson
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