Brain teaser lovers have been given a tricky which only people with a are likely to crack in less than 30 seconds.
Called the ‘rickety bridge’ test the challenge is to work out the quickest time it will take for four people to cross with one torch between them at night.
Only two people can cross at a time and they take differing times while the torch is always necessary to be carried as it would be too dangerous to cross otherwise.
The has been published by Geeks for Geeks organisation which also points out from the start the answer is not the one which will probably come to mind immediately with the four people crossing the bridge at times of: 1min, 2mins, 7mins and 10mins.
And so many people would say 21 minutes if the quickest person goes back and forth carrying the torch. But the organiser of the teaser wrote: “The initial solution most people will think of is to use the fastest person as an usher to guide everyone across.
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"But it would take longer as 10 + 1 + 7 + 1 + 2 = 21 mins. But can it be the right answer? No. That would make this question too simple even as a warm-up question.” Instead we are told that the answer lies in getting the slowest pair - who take 10mins and 7mins to cross together and the actual quickest time is 17mins.
And the way to do that is to use the quickest person to head across first with the second quickest and then the fastest remains on the other side while the other returns with the torch for those who take 7mins and 10mins to cross. At that point the fastest can then return with the torch.
explains: "Let’s brainstorm a little further. To reduce the amount of time, we should find a way for 10 and 7 to go together as they are the slowest among all these. If they cross together, then we need one of them to come back to get the others. That would not be ideal. How do we get around that? Maybe we can have 1 waiting on the other side to bring the torch back. This brings us closer to the solution. So let’s put all this together.
"Steps: 1 and 2 cross the bridge and move to the other side. Now 2 comes back with the torch from the other side. 7 and 10 crosses the bridge and 2 remain to this side only. Now 1 comes back with the torch from the other side. At last, 1 and 2 cross the bridge and we are done. Total time taken = 2 + 2 + 10 + 1 + 2 = 17 mins.”
