mad bomber

i've seen this puzzle quite a few places see who can solve it first?

a mad bomber is out on the job, making bombs. he has two fuses (pieces of string) of varying thickness which each burn for 30 seconds. unfortunately he wants this bomb to go off in 45 seconds. he can't cut the one fuse in half because the fuses are different thicknesses and he can't be sure how long it will burn. how can he arrange the fuses to make his bomb go off at the right time?

Originally posted by kcams
he can't cut the one fuse in half because the fuses are different thicknesses and he can't be sure how long it will burn.

Technically, he could just cut one fuse in half. If you already know that they take 30 seconds to burn completely, half is half no matter what.

Are there any other more specific clues? Like, cutting the fuses isn't an option at all? Or pre-burning the fuses isn't an option?

Don't need to cut or pre-burn.

Just lay the fuse attached to the bomb on the ground then lay the other fuse over it, perpendicular, bi-secting the second fuse. Basically creating a ' T ' with the two fuses.

Then when you light one end (a short end of the 'T'😉, it will burn half way (15 sec) and ignite the main fuse(30 sec).

Originally posted by dxzero
Technically, he could just cut one fuse in half. If you already know that they take 30 seconds to burn completely, half is half no matter what.

Are there any other more specific clues? Like, cutting the fuses isn't an option at all? Or pre-burning the fuses isn't an option?

half is not half no matter what, the fuses thickness varies along the length, this is what it states in the puzzle! that is what varying thickness means!

the solution is he burns 1 fuse from both ends this will definitely burn for only 15seconds no matter what (though the the two ends will not reach the middle but some other point [back to the varying thickness]) then as soon as the first fuse is burned he lights the second fuse. 15 + 30 = 45.

Originally posted by Saint Nick
Don't need to cut or pre-burn.

Just lay the fuse attached to the bomb on the ground then lay the other fuse over it, perpendicular, bi-secting the second fuse. Basically creating a ' T ' with the two fuses.

Then when you light one end (a short end of the 'T'😉, it will burn half way (15 sec) and ignite the main fuse(30 sec).

again this solution relies on an even thickness and equal burn which the original problem doesn't allow for.

see previous post for ellegant solution.

Originally posted by kcams
half is not half no matter what, the fuses thickness varies along the length, this is what it states in the puzzle! that is what varying thickness means!

the solution is he burns 1 fuse from both ends this will definitely burn for only 15seconds no matter what (though the the two ends will not reach the middle but some other point [back to the varying thickness]) then as soon as the first fuse is burned he lights the second fuse. 15 + 30 = 45.

This is the usual answer, but how would you make it work in a bomb? Presumably the bomber wants to light the 45 sec 'fuse' and then run away.

the problem is that the first fuse wouldn't finish in the middle, what about burning the first fuse on a set of scales so that when it has finished burning the mass has reduced or the heat suppplied in total moves a trigger for igniting the second fuse?

Pre burning is the answer. Burn one fuse for 15 seconds and put it out. This leaves you with a 15 second fuse which you attach to the 30 second fuse.

Originally posted by kcams
half is not half no matter what, the fuses thickness varies along the length, this is what it states in the puzzle! that is what varying thickness means!

the solution is he burns 1 fuse from both ends this will definitely burn for only 15seconds no matter what (though the the two ends will not reach the middle but some other point [back to the varying thickness]) then as soon as the first fuse is burned he lights the second fuse. 15 + 30 = 45.

I think your orginal problem was not very clear.
I took "varying thickness" to mean the two fuses were different thicknesses from each other. I think many others read it this way as well.

The solution you give here would not sove the problem, as the bomber would have to determine exactly where on the fuse "15 seconds" is. According to the varying thickness, he can not determine the "15 second" location on the fuse.

I think he would have to go with the pre-burning solution.

the solution is clear burning from both ends at the same time will mean that the fuse will definitely burn for only 15 seconds, the pre burning solution completly misses the point of the exercise.

it is suppose to be a simple puzzle and you've looked to deeply.

in future i'll try and be clearer in the first instance but with a riddle if it seems to obvious it probably is.

You cannot burn a varying length fuse from both ends and have it light the second fuse without human interference. How does that fufill the problem? The bomber wants the bomb to go off after 45 seconds. Under your answer he lights both ends of the first fuse waits till it is completely burnt (15 seconds) then lights the second fuse. The bomb will then go off 30 seconds later. However the bomber has to be with the bomb till after the second fuse is lit only giving him 30 seconds.

If you set the problem as measuring 45 seconds of time using the fuses then the problem makes sense.

Could the answer be to tie/splice/otherwise attach the fuses together? e.g if |--------------| is a 30second fuse, linking the two like this:

|---------------| "A"
_______|---------------| "B"

Will have "B" burning for 30 seconds, but after the first fifteen it ignites "A" that also burns for 30 seconds.

The problem with that is that since the fuses are not even in thickness down their lengths, you have no way of knowing how far down the fuse is 15 seconds.

Why do you think thickness would effect burning time?

Originally posted by AThousandYoung
The problem with that is that since the fuses are not even in thickness down their lengths, you have no way of knowing how far down the fuse is 15 seconds.

He only said uneven thickness, I assumed that meant just two thicknesses, e.g. on 4mm thick, one 5mm. You can't buy fuses that have widly varying thicknesses for the entire length.

Edit - ne'er mind, I only read the original post, not the third one.