Gordian Knot

[caddyman]

Rooting around in the dusty recesses of my office computer, I discovered a document containing the old U2 brainteaser, supposedly set by Micro$quash as a recruiting test, thus:

Allegedly, this is one of the questions for potential Microsoft employees. I must warn you, you can get really caught up in this problem. Reportedly, one guy solved it in two minutes. Another guy solved it by writing a C program - although it took him 37 minutes to develop it - the program solved it first go. A group of 50, at Motorola, couldn't figure it out at all. See how long it takes you!

OK here we go...

U2 (the band) have a concert that starts in 17 minutes and they must all cross a bridge to get there. All four men begin on the same side of the bridge. You must help them across to the other side. It is night. There is one flashlight. A maximum of two people may cross at one time. Any party who crosses, whether one or two people, must carry the flashlight with them. Thus the flashlight must be walked back and forth and cannot be thrown.

Each band member walks at a different speed. A pair must walk at rate of the slower man's pace:

Bono: 1 minute to cross
Edge: 2 minutes to cross
Adam: 5 minutes to cross
Larry: 10 minutes to cross

For example: If Bono and Larry walk across first, 10 minutes have elapsed when they reach the other side. If Larry then returns with the flashlight, a total of 20 minutes have elapsed and you fail the mission.

You must get all 4 across in 17 minutes or less. There are two known solutions to the problem.

Note 1: There is no 'trick' behind this - it is the simple movement of resources in the appropriate order.
Note 2: Microsoft expects you to complete this in up to 5 minutes!

It strikes me that the sane solution would be to just cut the rope bridge, stranding the band and then use the concert receipts to fund some serious counselling for the audience. But that’s probably just me and my dislike of Mr. Paul (second coming) Hewson. You may know him as Bonio, and I for one would be happy to let a pooch gnaw at him if it gave said mutt a glossy coat.

If you’re interested, the solution is
The trick is to get the two slowest people to cross at the same time. One solution is...
 Bono and Edge cross the bridge for which they take 2 mins (Total time = 2)
 Then Bono comes back with the torch (Total Time = 2 + 1 = 3)
 Then Adam and Larry cross the bridge (Total time = 3 + 10 = 13)
 Then Edge comes back (Total = 13+2 = 15)
 Then both Bono and Edge cross the bridge (Time = 15+2=17)

Comments

[fractalgeek.livejournal.com]

There must be at least 3 sets of crossings and 2 returns.

Being told it is 17 is really useful, as it means the most obvious solution, bono returns every time CANNOT work (it gives you 19). OK, then Bono must only return ONCE and can't cross with everyone. Working backwards, next best is return of 1+2, leaving 14 to go. Any other combination than Adam and Larry as a pair doesn't work.

Therefore Bono + Edge go; either returns
Adam and Larry go, the other from Bono and Edge returns
Bono and the Edge go again.

2 solutions

2 + 1or2
10 +2or1
2
=
17

Easy - since I know what the answer should be. Don't give me the constraint, and it would have taken MUCH longer.

[ed-fortune.livejournal.com]

Cudgel them all to death with the torch. Put them in a wheelbarrow. Should be less than 17 minutes, all told.

[bluesman.livejournal.com]

Or even cudgel them to death with the torch and jolly-well leave them there. Get a band that doesn't suck goo to play at the concert.

[w00hoo.livejournal.com]

I don't know U2 that well, but I'm guessing from that that Larry is the drummer, right?