Riddle: 5 lights

Here’s the new profile riddle:

A room has four lights. Alone, and unable to see in, you use four light switches outside the room and enter, knowing which is which. How?

At request, new riddles will be whittled down to 140 characters.

Upgrade to 5 lights

The riddle has so far received no correct answers (several people responded with the obvious “cop out” answer). Therefore, this is being declared the hardest riddle yet. I am upgrading the bounty to 2 points for a correct answer! Also, I am making the riddle a little harder…

You are outside a room with 5 light switches. You cannot see in the room. You don’t have anyone else to help you. You enter the room. Inside the room are 5 lights. You then find out which light switch controls which light.

This is very difficult. So remember, you are encouraged to Google around for similar riddles and to ask your friends. Cheating is encouraged!

SEND ME YOUR ANSWERS!!

The answer

And here, finally, is the “correct” answer:

While outside the room, you use the switches. You turn 1 and 2 on, 3 off. Then you turn switches 4 and 5 on and off, on and off, on and off very fast for 10 minutes. Finally, you set switch 2 and 5 off and 4 on:

Then you enter the room and look for the light that is off, but hot – this is light 2. Find the light that is on – this is light 1. Remove the remaining bulbs from the sockets and jiggle them. Two of them have burned out – the other one is light 3. Put light 3 into one of the remaining sockets. If it lights up, this is light 4, if it doesn’t, this is light 5.

Nobody got this answer. However, one alternate answer was accepted (+2):

Turn on lights 1, 2, 3 and turn off light 4. In two minutes, turn off light 2. After another two minutes, turn off light 3. Enter the room to find one light on, one light warm, one light very hot and one light off.

• Su

This answer was given just when the problem was being updated, so I took it (even thought is handles 4 lights). The best part is that since this answer is so different, it can be combined with my answer above to solve the 6 lights riddle.

Duncan put up the following answer which did start with the “three light riddle”:

You have 4 switches (A,B,C,D). Turn on switches A and B for about 5 minutes. Turn off switch B, turn on light D, and enter the room.

A=(Hot, On) B=(Hot, Off) C=(Cold, Off) D=(Cold, On)

This only works if you know how the off and on positions correlate to the lights.

I’m assigning this (+1, -1) points because I don’t like the assumption that lights don’t get hot immediately when on. But I do like the assumption that up is on, down is off (especially since when you enter the room you could conceivably find no light switches, implying that these are two-way switches and not three-way switches).

Also, many people tried the cop-out answer I was expecting:

The switches are labeled.

I was prepared to give points for this if nobody produced a “real” answer. So don’t get mad at me, get mad at her.

The scoreboard since 2002

* denotes a recent win/loss. (aa) denotes an alternate answer (huge win).

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Comments

You have 4 switches (A,B,C,D). Turn on switches A and B for about 5 minutes. Turn off switch B, turn on light D, and enter the room. A=(Hot, On) B=(Hot, Off) C=(Cold, Off) D=(Cold, On) This only works if you know how the off and on positions correlate to the lights.

Unknown

I'm not sure if this counts as a cop-out answer (and this obviously wouldn't work with CFL bulbs, but then again neither would the 4-bulb solution) but: Chose 1 switch. Flick it on and off continuously for a good, long while (say, a few days), or alternatively leave it on for a very long time (like, a year or two). The point is that the problem specifies no time limit. Once you've done that, then with the remaining four switches do the same thing as with the four-switch problem. Turn two switches on for a few minutes, then turn one of them off and turn another on. Quickly enter the room and feel the bulbs. You should be able to differentiate the last four bulbs between on/hot, on/cold, off/hot, off/cold as with the four-switch problem. The fifth bulb, the one with the broken filament, is the first one you chose.

Tyler McHenry

Thank you for your submission. I saw a another solution like from Duncan this and considered whether the underlying assumption is valid: after turning on, a bulb does not get hot immediately. Previously I thought this assumption fails and even tested it. However, now I see that for lower watt bulbs it is true! Naturally, valid assumptions are given to contestants, and Duncan retroactively gets his point. You, sir, are however WAY past the deadline. FYI the cop out answer was: the switches are labeled.

William Entriken

Please discuss this topic anywhere and let me know any great comments or media coverage I should link here.