Engineer's Puzzle

The Puzzle: The team bonding bridge bamboozler

Ashley Nicklin, 09/12/2015
I'm not an engineer or mathematician but I do have a brain ""allegedly"". Adrian’s 14 min’s I ...  Read More

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Four engineers are on a team building exercise, camping and navigating in remote countryside. All is going well, there are laughs and guffaws a plenty. However, towards the end of the night things take a turn for the worse. The weather turns in and they take to the tents to wait it out.

However, it’s not long before they realise the food supply has been savaged, presumably by the hungry looking badger they saw earlier that day. And to make matters worse, the water supply is near finished.

With mobile phones not allowed on the exercise, they decide to pack away and head back to civilisation. They come across a rickety old rope bridge, the kind Indiana Jones would look comfortable on, but not these engineers. They have one torch between them and shine it out over the bridge. It looks as if there are wooden slats missing at random and the weather gets increasingly bad.

They decide that for safety only a maximum of two people can be on the bridge at any one time. And the torch must be used by those crossing. The bridge is too long to throw the torch back so it must be returned by hand.

With fatigue setting in and with varying degrees of fear, the engineers are able to cross the bridge in 10mins, 5mins, 2mins and 1min respectively (How do they know this? Just go with me here...)

Piggy backing of any kind across the bridge will result in certain death, as will any kind of conga-style crossing. And the pairs must cross at the pace of the slowest person.

The question is, what is the quickest time all four can cross the bridge in safety, and how do they do it?

>A twist on the classic, ‘fox, chicken and grain getting across the river on a boat’, conundrum. Strangely, it is remarkably easy to get confused here, but hopefully the brainy power of you lot took less time to work this out than it took the engineers to cross.

The answer is 17 minutes.

First goes 2mins and 1min. Then 1min comes back with the torch. Total time so far 3mins.

Then 10mins and 5mins go across. They hand the torch to 2mins who crosses back over the bridge. Total time taken 15mins.

2mins and 1mins cross the bridge to safety.

They high five, group hug, and walk to safety feeling bonded by the ordeal.

Too easy? Well, why not email your puzzles to me at jcunningham@findlay.co.uk and put your wits against the might of the Engineering Materials readership.
Justin Cunningham

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Engineer's Puzzle Entries
I'm not an engineer or mathematician but I do have a brain ""allegedly"".
Adrian’s 14 min’s I believe is incorrect. If a person can cross in the dark then the answer is 11 mins. 10&5 cross, 5 flashes when reaching the other side, 2 starts walking. When 10 has crossed 5 flashes again and 1min starts walking (no more than 2 on the bridge).
Justin’s answer is almost correct. The 2 premises are that only 2 people may cross at any one time and you need the light to cross safely. Therefore in his case repeat as per Julian’s answer but the person holding the light can stop when the light reaches the far side on the first pass with the other completing the pass or starting! 1min walks back and when the light reaches the others pass the 10min guy starts. Torch is passed over 1min gets to side then 5 starts. 2mins starts as 10 finishes takes torch from 5 and turns when light reaches 1min guy. Both turn to walk back.
Therefore correct time is 17mins minus 4 lengths of time (4 crosses). The distance that can be travelled in the light by 1min, 2min and 10 min guy (5 min guy catches up to 10 who takes the torch on 3rd pass). To know the actual correct time one would need the distance of the light beam.


Comment Ashley Nicklin, 09/12/2015
10 and 5 cross - 10min, 5 takes it back to meet 1 coming the other way (only two crossing bridge using the torch), they meet about 80% across, 5 passes over torch to 1 and both return, so 1 is back with torch after total of 12min. Then 1 and 2 cross, total of 14 minutes.

Comment Adrian Jones, 25/09/2015
19 mins
The guy who can do it in 1 min returns the torch twice


Comment Miss Leigh Frazer, 22/09/2015

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