Scratch

greenflash · 2009-06-28 14:22:51

Here is a puzzle I found recently. Here is the situation:
Four people are trying to cross a bridge over a river at night:
1. They only have one flashlight, and they cannot cross the bridge without it.
2. All of the four people cross at different speeds.
a. The first person crosses in one minute, the second person crosses in two minutes, the third person crosses in five minutes, and the fourth person crosses in ten minutes.
b. If two people cross at the same time, they must walk at the speed of the slower person (So if the first person and the second person walk together, they will cross the bridge in 2 minutes, the speed of the second person, the slower one)
3. Only two people can cross at a time

What it the least time it takes for everyone to cross?
Refer to the first person as A, the second as B, the third as C, and the fourth as D.

Post your solutions!

Time to Match: 17 minutes

Game Version

Last edited by greenflash (2009-07-01 14:36:34)

Mozaz · 2009-06-28 14:39:42

Hm.. Taking it that only 2 people can cross at the same time:

The 3rd and 4th person cross first - 10 minutes
The 3rd person walks back - 5 minutes
The 3rd person and the 2nd person cross - 5 minutes
The 2nd person walks back - 2 minutes
The 1st and 2nd person cross - 2 minutes

= 24 minutes.

The 4th and 1st person cross first - 10 minutes
The 1st person walks back - 1 minute
The 3rd and 1st person cross - 5 minutes
The 1st person walks back - 1 minutes
The 1st and 2nd person cross - 2 minutes

= 19 minutes. Can't think of anything faster than that.

greenflash · 2009-06-28 15:10:45

Believe it or not, there is a faster solution, but you arer really close!

Mayhem · 2009-06-28 17:03:16

17 minutes.

Won't give it away but will say that you shouldn't have the fastest guy and the slowest guy cross together...

iamrpk · 2009-06-28 18:16:37

A and B go = 2 minutes
A comes back = 1 minute
A and C go = 5 minutes
A comes back = 1 minute
A and D go = 10 minutes

Total: 19

NOOOOO!

Last edited by iamrpk (2009-06-28 18:16:48)

GoogleplexPlus · 2009-06-28 19:40:13

D walks across. Tosses flashlight over bridge. Flashlight falls into water. A swears.

greenflash · 2009-06-28 21:45:40

You can't toss it. And 17 minutes is the solution. But how?

The-Whiz · 2009-06-28 22:04:41

Hm...

A and B
A
A and C
A
A and D

19!...

[Deleted due to giving away]
17!

Mayhem · 2009-06-29 02:04:46

greenflash wrote:
You can't toss it. And 17 minutes is the solution. But how?

1 minute guy and 2 minute guy cross - 2 minutes
1 minute guy returns - 1 minute
10 minute guy and 5 minute guy cross - 10 minutes
2 minute guy returns - 2 minutes
1 minute guy and 2 minute guy cross - 2 minutes

2+1+10+2+2=17

benjamin2 · 2009-06-29 13:37:05

A and B cross

CD_______________________________AB FLASHLIGHT
2 minutes

A goes back
FLASHLIGHT CDA_____________________________B
3 minutes

C and D cross
A_______________________________CDB FLASHLIGHT
13 minutes

B goes back
FLASHLIGHT AB_____________________________CD
15 mniutes

A and B cross
________________________________ABCD FLASHLIGHT
17 minutes
There you go!

greenflash · 2009-06-29 18:53:50

Good Job! If you still want to figure it out, you can, just don't look at pervious solutions!

Mayhem · 2009-06-30 15:16:23

You should turn this into a scratch project.

greenflash · 2009-06-30 15:25:19

yeah, i'm working on it! It's a bit delayed though, there's a glitch

greenflash · 2009-07-01 14:37:41

Ok I finished the project for this. You can see it at: http://scratch.mit.edu/projects/greenflash/590328

Mozaz · 2009-07-02 12:07:57

Aww.. thought I had it on the second go :@ Nice puzzle big_smile

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archived forums

#1 2009-06-28 14:22:51

Crossing the river at night (Logic Puzzle)

#2 2009-06-28 14:39:42

Re: Crossing the river at night (Logic Puzzle)

#3 2009-06-28 15:10:45

Re: Crossing the river at night (Logic Puzzle)

#4 2009-06-28 17:03:16

Re: Crossing the river at night (Logic Puzzle)

#5 2009-06-28 18:16:37

Re: Crossing the river at night (Logic Puzzle)

#6 2009-06-28 19:40:13

Re: Crossing the river at night (Logic Puzzle)

#7 2009-06-28 21:45:40

Re: Crossing the river at night (Logic Puzzle)

#8 2009-06-28 22:04:41

Re: Crossing the river at night (Logic Puzzle)

#9 2009-06-29 02:04:46

Re: Crossing the river at night (Logic Puzzle)

greenflash wrote:

#10 2009-06-29 13:37:05

Re: Crossing the river at night (Logic Puzzle)

#11 2009-06-29 18:53:50

Re: Crossing the river at night (Logic Puzzle)

#12 2009-06-30 15:16:23

Re: Crossing the river at night (Logic Puzzle)

#13 2009-06-30 15:25:19

Re: Crossing the river at night (Logic Puzzle)

#14 2009-07-01 14:37:41

Re: Crossing the river at night (Logic Puzzle)

#15 2009-07-02 12:07:57

Re: Crossing the river at night (Logic Puzzle)

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