IQ follow up

[Demosthenes]

Since so many people were labeled as Precision Processors or Visual Mathematicians, I was asked a rather simple question not too long ago. I rather disdain these sort of "who cares" mathematical quandaries, but, it sparked my interest. It took me more thinking time than it should to find the answer. See if you can do it.

Four guys want to cross a bridge in the dark of night. They have one flashlight and the bridge can hold only two people at once (do not ask why). Their walking speeds allow them to cross in 1, 2, 5, and 10 minutes (again, do not ask why). Show it is possible for them to all cross in 17 minutes in such a way that no one walks in the dark or prove that it cannot be done.

No fair if you saw the problem before. And you Google warriors out there, no cheating. Also, one example does not a proof make. I know what your first instinct is, and it is wrong. Trust me. Think it over before someone posts the answer and ruins the fun.

Respectfully,

Demosthenes

[Nuclear Krusader]

I think it can't be done.

But I look forward to the correct answer.

[Trent Steel]

I was wrong.

[Byte 2.0]

The slowest 2 start.
when the 2nd slow reaches the end the next slowest starts. when he reaches the end 2 will have cross, one will be on the bridge and the last guy come across, When He finished just before the main last slow guy. Then all 4 are across. You figure out the flash light.

I can get them all cross with in 10 minutes that way So you have 7 minutes to figure out a way to use the flashlight.

---------------------------------------

ok, the 10 minute guy and the 5 minute guy start.

Five minutes later then 5 minute guy finishes, the 10 minute guy is half way.

The 3rd guy starts, 2 minutes later he is finished. 10 minute guy is still on the bridge

The 1 minute guy starts, 1 minute later he finishes. 8 minutes have passed. Then 2 minutes later then 10 minute guy finishes.

all 4 are across.

Now the flash light. Ok, the 10 minute guy was really fast, he carried the flash light and run back and forth carring it to guide his friends that were carrying the beer. It ended up taking him an extra two minutes at the end because he carried the last cooler at the end and when the 3rd guy steped off the bridge the 10 minute guy opened the cooler and grabbed a beer before finishing the trip. He did this at the end of the bridge close enough that the 3rd guy did not get out of the light of the flash light.

[morriswindgate]

You give the flashlight to the slowest walking parson, then starting with the fastest walker the others start walking across the bridge.

[morriswindgate]

Better one. Two people start at ten feet from a wall. The first will walk halfway, stop and walk halfway again, and so on until he gets to the wall. The other person will do exactly the same. The first person walks at roughly 1 1/2 times as fast as the second person. Who will get to the wall first.

[M. A. Dockter]

1+2 walk with flashlight (2mins)
Send 1 back with flashlight (1min)
10 and 5 go with flashlight (10mins)
send 2 back with flashlight (2mins)
1 and 2 go across with flashlight (2mins)


2 + 1 + 10 + 2 + 2 = 17minutes


Most problems like this require you to overcome your instinct (to have 1 go with 5 or 10 on the second time across the bridge)

[homer15]

Originally posted by morriswindgate Better one. Two people start at ten feet from a wall. The first will walk halfway, stop and walk halfway again, and so on until he gets to the wall. The other person will do exactly the same. The first person walks at roughly 1 1/2 times as fast as the second person. Who will get to the wall first.

morris, did you make a typo or am i reading it wrong...

[doctorgonzo]

Neither will ever get there. Zeno's paradox.

[Byte 2.0]

Zeno's paradox.

That is because you keep deviding the remaining distance by half, so you never get there right?

[M. A. Dockter]

Originally posted by byte Zeno's paradox. That is because you keep deviding the remaining distance by half, so you never get there right?

Correct

[kittyfire]

I just can't help but point out that girls don't have problems like that.

[homer15]

the xena paradox applies to the ladies.

[Demosthenes]

The flashlight is there for a reason. An assumed constraint is that no one can walk in the dark. I should have explicitly told you that, but the problem is pretty trivial if you do not. ;)

Well, since someone posted the answer, here is a better problem I was asked after the bridge one. It is harder and I could not give an answer on the spot.

Imagine a 6x6 checker board. If there are 18 dominoes of 2x1 size (that is, they cover exactly two adjacent squares, then it is possible to completely cover the board with dominoes. But, an indirect result of this is that for all ways of covering the board, there exists a horizontal or vertical cut that will split the board into two pieces leaving all dominoes intact. Do you know why?

A simple follow-up, is this true on an 8x8 board?

These are the so-called "Microsoft Interview" questions which they can and do indeed ask their applicants.

Respectfully,

Demosthenes

[kittyfire]

I'd rather yodel "lelelelele" and turn flips in the air than try to figure out those problems. lol

[Black Ice]

Ok you like brain teasers I spend a lot of time here check it out.

http://www.mensa.org/workout2.html?

[homer15]

hey bobby, does that get updated?

[Black Ice]

No it hasent been updated for a long time.But most of the links have. To get to the good stuff you must join . I did it for a year then droped out it was kinda boring for me.