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4 women who want to cross a bridge

There are 4 women who want to cross a bridge. They all begin on the same side. You have 17 minutes to get all of them across to the other side. It is night. There is one flashlight. A maximum of two people can cross at one time. Any party who crosses, either 1 or 2 people, must have the flashlight with them. The flashlight must be walked back and forth, it cannot be thrown, etc. Each woman walks at a different speed. A pair must walk together at the rate of the slower woman's pace.
Woman 1: 1 minute to cross
Woman 2: 2 minutes to cross
Woman 3: 5 minutes to cross
Woman 4: 10 minutes to cross

For example if Woman 1 and Woman 4 walk across first, 10 minutes have elapsed when they get to the other side of the bridge. If Woman 4 then returns with the flashlight, a total of 20 minutes have passed and you have failed the mission. What is the order required to get all women across in 17 minutes? Now, what's the other way?

 

Add your possible solutions in a comment, I will put the one which will convience me here


Comments:

Women Crossing
By kollol on Sunday, August 20, 2006 (UMST)

1 and 2 go and 1 comes back. Total Time= 3 mins

 

then 3 and 4 go and 2 comes back. Total Time in this = 12 mins

 

lastly 1 and 2 go. Total Time: 2 mins

 

Net total time to cross= 3+12+2= 17 mins

Reply to this Comment

Read the problem again please
By Aboelnil on Thursday, November 09, 2006 (UMST)
Values are 1,2,5,10 now 1,2,3,4

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Read the solution again, please.
By Celton on Thursday, December 07, 2006 (UMST)
Kollol has it right.  Now, what is the other way?

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The other way
By Guild on Tuesday, December 19, 2006 (UMST)

I think the other way is just a variation of the first, with 2 going back with the flashlight instead of 1:

 

1 and 2 go and 2 comes back. Total Time= 4 mins

 

then 3 and 4 go and 1 comes back. Total Time in this = 11 mins

 

lastly 1 and 2 go. Total Time: 2 mins

 

Net total time to cross= 4+11+2= 17 mins

 

Reply to this Comment

This might be it
By Prasida on Friday, July 20, 2007 (UMST)

I think kollol is wrong since  -

 

There is one flashlight. A maximum of two people can cross at one time. Any party who crosses, either 1 or 2 people, must have the flashlight with them. The flashlight must be walked back and forth, it cannot be thrown, etc.

 

So, according to kollol,

 

1 and 2 go and 1 comes back. Total Time= 3 mins

 

then 3 and 4 go and 2 comes back. Total Time in this = 12 mins
(Where is the flashlight??)

 

I believe, this is how it wormust work -

 

3 and 4 walk with the flashlight. Time - 10 mins.

3 comes back with the flashlight, while 1 begins to walk toward the destination simultaneously. When 3 reaches a distance of x/5 (let x be the total length of the bridge), 1 reaches the destination. Time - 1 min.

Now, 3 turns and walks back towards the destination, while 1 walks towards the origin simultaneously. They exchange the flashlight on the way, so now 1 has the flashlight. When 3 reaches the destination, 1 would have reached the origin. Time - 1 min.

lastly, 1 and 2 come back. Time - 2 mins.

 

Hence, total time = 10+1+1+2 = 14 mins

 

The other way,

 

3 need not turn back but can continue the journey towards the origin, while 2 now starts the journey. Here the time taken would be less than 5 mins. Hence, the total time would be less than 17 mins.

 

 

 

Reply to this Comment

Really?
By speck on Tuesday, October 30, 2007 (UMST)
First of all, in kollom's solution, the flashlight is always in motion because #1 brings it back after the first trip across, handing it to the pair of #3 and #4 for their slow trip across. the flashlight is now on the far side, where #2 can bring it back to the beginning to pick up #1 again. Having people start out from either side to meet in the middle is obviously out of the question because one or the other has to be walking without the flashlight.

Reply to this Comment

AreYou Sure???
By misztiffany on Wednesday, January 23, 2008 (UMST)

Well Prasida, Kollol is correct.

 

There is only person holding the flashlight, but it doesnt that person holding it cant give it to the next two people going does it now???

 

So 1 and 2 goes and then 1 comes back with the flashlight. (3 mins)

 

1 gives the flashlight to 3 or 4,  then 3 and 4 goes and hand it to 2 (10 mins)

 

2 comes back with the flashlight (2 mins)

 

And then 1 and 2 goes. (2 mins)

 

Easier understanding:

 

1 & 2 goes = 2 mins

 

1 comes back= 2 mins

 

3 & 4 goes = 10 mins

 

2 comes back= 2 mins

 

1 & 2 goes= 2 mins

 

Total mins is 17 mins...

 

Reply to this Comment

Only 12 Minutes
By James Bond on Wednesday, March 19, 2008 (UMST)
The problem said I "have 17 minutes to get all of them across to the other side." Therefore, there are five people in this operation, not four. I'm very speedy, can cross the bridge faster than the speed of light, and waiting at the other side. Women 3 & 4 cross, I run the flashlight back and hand the flashlight off. 10 min. Women 1 & 2 cross - 2 min. 12 minutes total. Beat that!

Reply to this Comment

About James Bond
By Cibert on Tuesday, July 08, 2008 (UMST)
Yep, I Think James Bond Had The Correct And Most Real Answer, So I Think We Lose =(.

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