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Old 08-01-2018, 02:09 AM
millie9 millie9 is offline
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Join Date: Aug 2018
Posts: 2
Default Need help in solving Cross the Bridge puzzle

Hi all,

I need help in solving the cross the bridge puzzle asked at interviews as asked here.

Four people are on this side of the bridge. Everyone has to get across.
Problem is that it’s dark and so you can’t cross the bridge without a flashlight and they only have one flashlight.
Plus the bridge is only big enough for two people to cross at once.
The four people walk at different speeds:

One fella is so fast it only takes him 1 minute to cross the bridge,
another 2 minutes,
a third 5 minutes,
the last it takes 10 minutes to cross the bridge.
When two people cross the bridge together (sharing the flashlight), they both walk at the slower person’s pace. What is the minimum time required for all 4 to cross the bridge.

Respond with a number which represents the number of minutes.

Thanks in advance.

Last edited by millie9; 08-01-2018 at 02:11 AM.
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Old 08-01-2018, 07:00 AM
Feelthecosmos Feelthecosmos is offline
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Join Date: Sep 2017
Posts: 9

Wait, you just want a number? Then 19.

Do you want the job? It's 20.

The most efficient way is to have your 1 minute person escort everyone across the bridge. So, 10 and 1 go across, then 1 goes back, 5 and 1 cross, then 1 goes back, 2 and 1 cross and you're done. 10+5+2 is 17 minutes, plus the 2 return trips is 19.

This is a job interview question though, so imagine these are your employees or fellow team mates. You're making the best worker take 5 TRIPS across that bridge, for the sole reason that they're better than their fellow teammates. If you're paying them equally, I hope you enjoy asking interview questions because you'll be looking for new people all the time.

If you want to keep your staff, send the 1 minute person over with the 10 minute person, then back across. Have them pass the flashlight to the 2 minute person, who can escort the 5 minute person, then come back with the flashlight. Then the 1 and 2 minute person go back together. Total time: 20 minutes, nobody can complain that they had to do more work than everyone else, the "slack" is shared by a colleague, and the fastest person gets a break every once in a while.

Throw an answer like that at your job interviewer and you show them that you know your stuff, and also prepare for things.

Last edited by Feelthecosmos; 08-01-2018 at 07:43 AM. Reason: gender neutrality - why's everyone gotta be a he?
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Old 08-01-2018, 11:24 AM
Feelthecosmos Feelthecosmos is offline
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Join Date: Sep 2017
Posts: 9

Haha, apparently afternoon me is a wee bit smarter than morning me. That response was intelligent, thoughtful, insightful and completely wrong.

I can do it in 17 minutes this afternoon. Anyone do better?

Send 1 and 2 across then send 1 back with the flashlight. 3 minutes.
Send 5 and 10 across, but send 2 back with the flashlight. 12 minutes, 15 total.
Send 1 and 2 across. 2 minutes, 17 total.

And, 1 and 2 make 3 trips each, so nobody is mad at each other. Come on morning me, get your game together!
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Old 08-01-2018, 12:22 PM
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uigrad uigrad is offline
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Join Date: Jan 2014
Posts: 114

There is a faster way!!

First, 1 and 2 travel together, and 1 comes back. Time = 2 + 1.
Next, 5 and 10 travel together! Now, 2 comes back. Time = 10 + 2
For the last trip, 1 and 2 travel together. Time = 2

Total time is 2 + 1 + 10 + 2 + 2 = 17. You saved two minutes by pairing 5 and 10 together.

Also, it should be noted that the work is evenly split between 1 and 2.

EDIT: I see that I'm giving the same solution as the second message from Feelthecosmos. For some reason, I didn't see that message before!

Last edited by uigrad; 08-01-2018 at 12:26 PM. Reason: Fixed
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Old 08-02-2018, 05:27 AM
millie9 millie9 is offline
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Join Date: Aug 2018
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@Feelthecosmos @uigrad Thank you guys for the help, 17 is the right number.
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