28 replies to this topic

[toddpeak]

How about this:
fold the first cord at the middle (you can make sure this is the case by making sure the opposite ends meet when folded), and light it up (such that both ends start burning at the same time)

While it is burning (and it should take 30 minutes) fold the other cord twice (so that you have it in quarters).. once the first cord finishes burning, light the other cord (such that all four bits start burning).. this should take 15 minutes, and you have a total of 45!

ok, so technically, you might argue that the parts might burn eachother, but you can always space them out such that they don't!

I don't think this would work. All four bits would have to be burning the whole time for it to work. However, one of the four might burn almost instantly leaving a different piece to burn for a pretty long time. The value in burning both ends of the rope is that you know the rope is being consumed twice as fast for the entire period. If one of the four bits burns faster than the others, then the rope is only being consumed at the pace of three flames instead of four. Since we're not sure how fast each bit will burn, we can't do it this way.

[weasel_coo]

While it is burning (and it should take 30 minutes) fold the other cord twice (so that you have it in quarters).. once the first cord finishes burning, light the other cord (such that all four bits start burning).. this should take 15 minutes, and you have a total of 45!

Not quite - but this did give me an idea.

If you burn the first cord at both ends then it will be consumed in 30 minutes - we all get that.

However, if you take the second cord and burn it at both ends AND somewhere in the middle so there are four flames on the cord then it will be consumed four times as fast so in 15 minutes. The only caveat here is that there has to be four flames on the cord at ALL TIMES. So if you start burning the cord in the middle and after two minutes one of the pieces is consumed completely, you have to start burning the other piece in the middle again.

It would take a bit more effort than the original solution but I think it would still work.

[danm]

1. Start the process with three cords

2. Ignite cord "a" on both ends and cord "b" on one end.

3. When cord "a" has burned out there is 30 minutes left in cord "b"

4. Ignite the second side of cord "b" and one end of cord "c"

5. When cord "b" has burned out (15 minutes later) there will be 45 minutes left in cord "c"

[danm]

Whoops, jumped here from another thread....didn't catch the two cord limit...

[arun]

It seems quite clear to me. Second condition says:
2. Every cord burns from ignition to the end exactly one hour.
So if you ignite the same cord on both ends, it should stop burning after half an hour. And, of course, spot where the fire from both ends meets does not have to be in the middle at all.

I shall submit my question very soon as i feel some flaw in the solution

Edited by arun, 29 May 2008 - 10:26 AM.

[themagicschoolbus]

fold the cord into 4 equal parts and wait till 3 of three of the four parts burn

[Martini]

fold the cord into 4 equal parts and wait till 3 of three of the four parts burn

That won't work. The riddle states, "different parts can burn at different speed".

[ktglo4him]

I believe this solution is also possible .. Please comment ...

1. Ignite the first cord at the two ends and somewhere approximately in the middle so that the fire from the middle propagates towards the two ends. It should take 15 minutes for this first cord to burn out completely (There would be two pieces of the first igniter cord since it was ignited at the middle as well - EVERY LAST BIT should burn out)

2. Ignite at the second cord at the two ends as soon as the first cord "completely" burns out. This would take another 30 minutes.

The only problem is that it would be almost impossible for you to find the middle point from which the cord is to burn equally in both directions. It might reach one end 5 minutes before it reaches the other (due to the uneven burning speed of the materials). Thus you would not be able to accurately measure 15 minutes.

[glamar]

For those struggling with the solution, think of it with some more data, this is not part of the puzzle nor does it need to be, but will help you wrap your head around this:

Lets say each cord is 10 units long and for whatever reason the burn times for each unit are 1,2,3,4,5,6,7,8,9,15 = 60 (in Minuets), folding is half will not get you 30 sec on each side. If you light both sides they meet 1/4 the way through the 8th unit. If you have two of the cords and you light three ends the one with both burning will be gone in 30 mins the other, depending on what end you light will have 7 1/4 units left or 2 3/4 units, either way they will = 30 mins. If you light the remaining end it will take 15 mins to burn out so the total burn time is 45 mins.

Not sure why any one would manufacture detonation cord like this :-). It there a real life application to this puzzle?

My two cents!
(Really just two coins not sure if they are both worth a cent but for whatever reason I know they = 0.02 of a dollar)