riddling day for donkeys.

[chuddo]

this is decent somewhat classic riddle that happens to be one of my favorites.

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There are 4 people who need to cross a narrow bridge at night with only one torch.

The four people each have different travelling times:

Person 1 crosses in 1 minute
Person 2 crosses in 2 minutes
Person 1 crosses in 5 minutes
Person 1 crosses in 10 minutes

Only two people can cross at a time and one person has to come back over the bridge to give the torch to the others still waiting to cross.

While crossing, you have to use the slowest time of the two people because they have to walk the same pace.

You have to add the person who comes back to your total time.

They need to cross in 17 minutes. How?

[imported_anacardo]

I'unno. Tough. Can't get it below 19 minutes.

Man 1 & man 4 cross - 10 minutes.
Man 1 returns - 11 minutes.
Man 1 & man 3 cross - 16 minutes.
Man 1 returns -17 minutes.
Man 1 & man 2 cross - 19 minutes.

I can't think of a more efficient combination than that.

[LBJ]

*******SPOILER/CORRECT ANSWER**********





Ok, I was the first person to click on this thread and have been thinking about this for about 19 minutes. I was about to give up and then I realized there had to be a logical way that didn't involve 1 keep going back and forth with 2, 5, and 10.

1 and 2 cross- thats 2 minutes
1 come back- thats 1 minute
5 and 10 cross-thats 10 minutes
2 comes back- thats 2 minutes
1 and 2 cross- thats 2 minutes

[Vavavoom]

[ QUOTE ]
Ok, I was the first person to click on this thread and have been thinking about this for about 19 minutes. I was about to give up and then I realized there had to be a logical way that didn't involve 1 keep going back and forth with 2, 5, and 10.

1 and 2 cross- thats 2 minutes
1 come back- thats 1 minute
5 and 10 cross-thats 10 minutes
2 comes back- thats 2 minutes
1 and 2 cross- thats 2 minutes

[/ QUOTE ]

What LBJ said...

[imported_anacardo]

Ah. It IS better for the slow-asses to get their [censored] done all at once. Good call. Pass that torch!

[imported_The Vibesman]

Send the 1minute guy and 2minute guy first. (2 min) 1 min guy goes back w/ torch (3 min) 5min and 10min go (13 min). 2min guy goes back w/ torch (15min). 1min and 2min cross(17 min)

[Patrick del Poker Grande]

Wait wait wait... I got it!

First, the 1 and 2 guys go and the 1 guy comes back. That's 2+1=3 minutes.

Next, send the 5 and 10 guys! Bring the 2 guy back. That's another 10+2=12 minutes. Total = 3+12=15 minutes.

Finally, you just need to bring the 1 and 2 guys back across. 2 minutes for a total of 15+2=17 minutes!

I win!

[Ghazban]

A friend of mine applied for a job right out of college with Microsoft and this problem was part of the interview process. They had to do 4-5 of these sorts of problems in some kind of short time span (1/2 an hour maybe? I'm not sure). This was the only one my friend couldn't figure out so he asked me and another math geek who lived in our house the following day and was irritated that we both figured it out pretty quickly.

Needless to say, he didn't get the job.