Baxil (baxil) wrote,
Baxil
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Riddles: More river crossings

Alright, I admit it: The last riddle was less of a brainteaser than it was a chance to snark.

I spent most of Friday juggling no less than three separate cars -- and two other people's errands -- in order to find a way up to my former house to pick up a clothes washer-dryer and haul it home. I was supposed to get a refrigerator, too, but simply ran out of time, and I'll have to deal with that this upcoming week (hopefully via U-Haul or a quick direct sale rather than by solving riddles just to find a vehicle capable of hauling it).

Anyway. Both of my previous riddles have been completely original, and so now I'm feeling pangs of guilt for posting such a time-worn one. It was wearing at me while I walked to work the next day, so I took some time to think up a new twist on the river-crossing theme:
#1. Alex, Ben, Charlie, Dan, and Eric are in a horror-movie plot out in the remote wilderness, being stalked by a serial killer that always murders his victim the moment that the two of them are together and nobody else is close enough to come to the victim's aid. The five survivors (the other 21 have been murdered) have figured out that the serial killer is secretly one of them, but they don't know who, so they've agreed to go back to civilization to contact police -- and all stick together so the killer won't strike again on the way.

Unfortunately, they come across a wide river filled with alligators. There is a single three-man boat on the bank. It would be easy enough to take two trips and ferry everyone across -- but if the serial killer is left alone on either bank with a victim, or is in the boat alone with a victim, they can murder one final person and escape.

"We'll just have to take our chances," Charlie says. "Without knowing who the killer is, all we can do is cross the river as efficiently as possible and hope nobody gets slaughtered. I call dibs on rowing the boat back from the first group to the second -- that way I'm never alone with the killer."

"Are you crazy?" Ben asks. "You can't call dibs on that. You're the one who suggested the trip to Dead Camper Lake in the first place. I call dibs because I suggested we go back to civilization together."

"Guys," Dave Dan interjects. "We know neither Alex or Ben are the killer, because back near the beginning of the trip we sent them off together to scout that deserted cabin. Let's leave the two of them here and the three of us take the boat across and go home."

"No," Eric said. "The deal was we all go back. I'd feel safer in a group of five anyway. And if I stick with Alex or Ben while you guys cross and someone comes back for us, I know I won't get murdered."

Alex thought for a moment. "Hang on. If you guys stop your squabbling, we can all get across without putting anyone at risk," he said.

How?


Also, as it turns out, amthrax proposed a different river-crossing tale with a similar solution in comments to the previous post ... so I'm going to present it here. Feel free to answer either or both.
#2. There are four people on one side of a river who need to cross the bridge to the other side. Of the four people, one of them is a runner who can cross the bridge in one minute, another is a brisk walker who can cross the bridge in two minutes, the third is a normal walker who can cross the bridge in five minutes, and the fourth person takes ten minutes to limp across. It's night-time and they have only one lantern.

The bridge is rickety and small, so only two people can cross at a time, and one of them must be holding the lamp so they can both see. When a pair of people is crossing, the faster person cannot leave the slower person behind in the dark, so the faster person must walk at the slower person's pace. How can you get all four people across in as little time as possible?


Hint: (The best solution + 4) is not a prime number.

As usual, comments will be screened for a few days to allow everyone to figure these out on their own. And as a reminder, if you never saw the solution to the dartboard problem, answers are visible now.

--

Edit, 8-17-05: Comments now unscreened.
Tags: riddles
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