30 August 2016
Engineer's Puzzle

The Puzzle: Blowing a fuse

Brian, 01/10/2016
I still don't understand the 30 minutes. One end burns out in 10 minutes, the other in 50. Perhaps ...  Read More

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Blowers, a demolition engineer, has just set up two rounds of explosives around two old mill towers. He needs to detonate the explosives exactly 45 minutes apart to bring them down as he wants. But, he soon realises he left his watch at home. He thinks… ???

There are some old flares in the van he just brought. ‘These could be of use…’ he thought. The flares burn for exactly one hour after they’re lit. But, the flares don’t burn at a constant rate. One end burns away in 10 minutes while the other end takes 50 minutes.
Blowers, knows what to do. He grabs his lighter…

The question is, how can he measure 45 minutes?

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Blowers sets off the first detonation, lights the first flare at both ends and another flare at one end… all simultaneously (go with me on this.) The first flare burns out in exactly 30 minutes. Blowers then knows that the second flare has exactly 30 minutes to go before it also burns out completely. But he doesn’t wait for that.

He lights the other end of the second flare. It means that the second flare will be burnt out completely after 15 minutes, which of course adds up to 45minutes!

Boom!

Too easy? Not satisfied with the answer? Leave your comments - smart assed or otherwise - below.

And, why not email your puzzle to justin.cunningham@markallengroup.com and put your wits against the intellect of the Engineering Materials’ readership.
Justin Cunningham

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Engineer's Puzzle Entries
I believe the answer provided is meant for another puzzle: there are two flares which burns out in 60 mins. How do you measure 45 minutes then?

The actual answer to the puzzle should be: Light up both ends of the 10-min flare to measure 5 minutes. Simultaneously, light up one end of the 50-min flare.

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Comment James, 07/06/2017
My solution would be cut the 50 min flare in half 25 min then use two 10 min flares

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Comment SAK, 17/10/2016
He cuts a 50min fuse in half = 25mins arranges them inline so 10min fuse 25min fuse 10mm fuse , then trigger the first detonation whilst lighting the first fuse when burnt lights the second then third, then detonation when finished burning.

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Comment simon, 17/10/2016
I am with Brian. Now neither the puzzle nor the explanation makes sense, plus I am annoyed I wasted time and brain-power on it !!!

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Comment peter, 07/10/2016
I still don't understand the 30 minutes. One end burns out in 10 minutes, the other in 50. Perhaps I simply don't understand flares!

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Comment Brian, 01/10/2016

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