The Grey Labyrinth is a collection of puzzles, riddles, mind games, paradoxes and other intellectually challenging diversions. Related topics: puzzle games, logic puzzles, lateral thinking puzzles, philosophy, mind benders, brain teasers, word problems, conundrums, 3d puzzles, spatial reasoning, intelligence tests, mathematical diversions, paradoxes, physics problems, reasoning, math, science.

Ghost Post · Icarian Member

Everyone can get across the bridge faster. The puzzle does not appear to require that the travelers remain together, only that just 2 may be on the bridge at once and all must cross before the 17 min elapses.
A&B cross together in 2 min
A returns in 1 minute
A & D depart together (D holds the flashlight) - after 1 minute, D is alone on the bridge, and C may begin. The flashlight is always on the bridge, so no one is crossing in the dark.
Everyone will have crossed the bridge in 13 minutes.

Murray · Daedalian Member

It's great to look for a better solution, but sometimes one doesn't exist. This is such a case. The problem clearly states that, "Crossing the bridge in the dark would certainly be fatal," i.e. the people crossing -must- be with the flashlight. At the very least, you can't assume that the flashlight will be able to illuminate the entire bridge. Also, your solution assumes that the crossers have telepathic powers that allow them to know when someone else has reached the other side of the bridge. Unlikely.

However, even given your relaxed reading of the problem, you have still not achieved the optimum time:

C and D begin crossing (D holds the flashlight). When C finishes (five minutes), B begins to cross and finishes in two minutes (seven minutes have elapsed). D is still on the bridge, and A begins to cross, finishing in one minute (eight minutes have elapsed). Finally, D catches up to the rest of the gang, and only ten minutes have passed. Voila.

But once again I must say that this is not at all a valid solution.

Ghost Post · Icarian Member

Come on now, the puzzle clearly states that there is a full moon so the flashlight is unnecessary and people can walk across without someone ever having to come back. Of course they still have to worry about making sure there is never more than two on the bridge.

the.cynic · Daedalian Member

The puzzle starts out with "One foggy night..." The full moon only brings out the goulies. Felicitous

wordcross · Guest

There is another form of this exact same puzzle that placed a requirement on several key aspects of this puzzle. The one in question was addressed by saying that the two had to be together at all times. Another disallowed the use of piggy backs or other such aiding maneuvers.

wordcross · Posted: Tue Nov 09, 1999 10:53 pm Post subject: 6

There is a variation on this puzzle that requires that the people be together at all times. Thus the required 17 minutes is all used. It also prohibited the use of piggy-backs etc.