Tunnel Crossing...

[imarshad]

Assalam u Alaikum,
Here is a classical riddle that has been asked quite a few times and yet it is not easy to solve for the new ones....

4 people have to cross a tunnel. There is darkness in the tunnel and they have to use a single flashlight. There can be at the most 2 people in the tunnel. It takes different time for different people to cross the tunnel.
Mr. A takes 1 min.
Mr. B takes 2 min.
Mr. C takes 5 min.
Mr. D takes 10 min.

How will they all cross the tunnel in min. time?

[Ibn Al-Jarrah]

take the train?

[imarshad]

take the train?
There is no train or other service.... They will have to cross by foot....

[Love&Peace]

Assalam u Alaikum,
Here is a classical riddle that has been asked quite a few times and yet it is not easy to solve for the new ones....

4 people have to cross a tunnel. There is darkness in the tunnel and they have to use a single flashlight. There can be at the most 2 people in the tunnel. It takes different time for different people to cross the tunnel.
Mr. A takes 1 min.
Mr. B takes 2 min.
Mr. C takes 5 min.
Mr. D takes 10 min.

How will they all cross the tunnel in min. time?

I don't understand the last statement?
here min. stands for minumum or 1 minute?

[imarshad]

I don't understand the last statement?
here min. stands for minumum or 1 minute?
It means minimum time....

[Mace]

This is a fun one. Should be some good discussions. :)

(I've heard this one before, so I'm not going to spoil it.)

[Islamisthebest]

umm B,C and D all get on A's back and ride lyk animals or something:S who knows im always wrong

[Islamisthebest]

wait it take that back it says 2 ppl at a tym ma bad, i cant read at this tym of the early hours of the mornin

[imarshad]

umm B,C and D all get on A's back and ride lyk animals or something:S who knows im always wrong
It is a mathematical question and has a real answer....

[Love&Peace]

According to me it should then take only 10 mins
the person hwho takes 10 mins should have flashlight in his hand... I assume Flash light has powerful flash to illuminate the whole tunnel, and by the time he is walking through tunnel every one else can pass the tunnel....

or may be I am wrong..

1. Is Flash light sufficient enough to illuminate the tunnel or if A and B is walking, they should walk the same path and hold the flash light?

If so then first 10min and one min can go together, 1min can come back. 1min and 5 min can go, 1 min can come back. Finally 1min and 2 min can go together.......

2. Can anyone sit on any one?

am I asking too many question .... Sorry :(

[Mace]

According to me it should then take only 10 mins
the person hwho takes 10 mins should have flashlight in his hand... I assume Flash light has powerful flash to illuminate the whole tunnel, and by the time he is walking through tunnel every one else can pass the tunnel....

or may be I am wrong..


No, the constraint is that only two people can be walking in the tunnel at a time. And the light is sufficient for only those two people walking together.



1. Is Flash light sufficient enough to illuminate the tunnel or if A and B is walking, they should walk the same path and hold the flash light?


Correct.


If so then first 10min and one min can go together, 1min can come back. 1min and 5 min can go, 1 min can come back. Finally 1min and 2 min can go together.......


Now you're getting the general idea.



2. Can anyone sit on any one?


It wouldn't make it any faster if they could. Only two people can be in the tunnel at once. They might as well both be walking. :)


am I asking too many question .... Sorry :(

And I hope the OP doesn't mind me answering them. :)

[Love&Peace]

Mace,
Thanks for the reply....

Now I want 10 and 5 to go together so they don't waste time going separately
and I want none of them to comeback ...

1 & 2 - 2min
1comes back - 1min
10 & 5 - 10mins
2comes back - 2mins
1 & 2 - 2mins

So minimum 17 mins..... someone pls suggest even lesser time :)

[Mace]

Mace,
Thanks for the reply....

Now I want 10 and 5 to go together so they don't waste time going separately
and I want none of them to comeback ...

1 & 2 - 2min
1comes back - 1min
10 & 5 - 10mins
2comes back - 2mins
1 & 2 - 2mins

So minimum 17 mins..... someone pls suggest even lesser time :)

I think you got it. In fact, you get all of these, don't you? :)

17 is the best I could come up with, too.

Going with Imarshad's naming of the people, you could have Mr. A or Mr B. return after the first crossing. It doesn't matter.

But yes, since you are going to take a hit on slow Mr. D, you might as well have Mr. C go with him on that segment.

[Ibn Al-Jarrah]

i knew that

[imarshad]

Mace,
Thanks for the reply....

Now I want 10 and 5 to go together so they don't waste time going separately
and I want none of them to comeback ...

1 & 2 - 2min
1comes back - 1min
10 & 5 - 10mins
2comes back - 2mins
1 & 2 - 2mins

So minimum 17 mins..... someone pls suggest even lesser time :)

Assalam u Alaikum,
Yes this is the correct answer....