I love a good brainteaser, so here — let's take a look at one from a recent TED-Ed video put together by Alex Gendler. In it, we're faced with a challenge: Can you solve the bridge riddle? The riddle itself has a similar setup to the word problems you used to get in school — y'know, the ones asking you to ferry a bunch of different animals across a river without leaving, say, the wolf alone with the chickens for fear that one would consume the other. This one, though? It's a lot more fun, because, well... you'll see. Ready? Here we go!
The Problem:
Picture this: Because you are a scientific all-star, you landed the world's greatest summer internship. So what if it is at a secluded lab all the way at the top of a mountain? What's the worst that could happen?
Uh… zombies. Zombies could happen, because in a moment of intense and regrettable curiosity, you made the mistake of pulling the lever by your work station marked with a skull and let out the hoards of undead your mentor, the Professor, has been keeping in the basement. You, your fellow Assistant, the Janitor, and the Professor all make a run for it — only to find yourselves confronted with a long, rickety bridge spanning a deep and possibly bottomless chasm. Obviously you need to get everyone safely over the bridge and cut the ropes before the zombies get to you — but there are a few catches.
- The zombies will catch up to you in just over 17 minutes, according to the Professor's calculations, so that's how long you have to pull off your daring escape.
- All your party members travel at different speeds. You can make the crossing in one minute; the Assistant can do it in two; the Janitor, who's a little slower, takes five minutes to do it; and the Professor takes 10 minutes, because he's ancient and that's what happens when you're ancient.
- The bridge can only hold two people at a time; otherwise it will buckle and collapse under the weight.
- You did manage to grab a lantern on your way out of the zombie-infested lab, but it's a poor substitute for a Maglite and can only illuminate a small portion of the bridge at a time. As such, anyone crossing the bridge either needs to be in front holding the lantern, or walking right behind the person who's carrying it. No one can cross the bridge without the aid of the lantern, because have you ever seen how dark it gets on a secluded mountaintop at night?
- Everyone has to be off the bridge before the zombies get to it, because two crossers plus one zombie equals three people on the bridge — which, according to condition number two, means death.
So: How do you get everyone safely across the bridge before the zombies get to you?
Want a second to figure it out? Go ahead; I'll wait. Just in case you need some encouragement, here is what is chasing you right now:
Think you got it? OK — let's check your work.
The Solution:
The key is to minimize the time taken by the two slowest people — the Janitor and the Professor — by having them cross together. You'll also need to make a few return trips to get the lantern back and forth, so you'll want the two fastest people (you and the Assistant) available for that. Here's how it's done:
Trip One: You + Assistant = Two Minutes
First, you and the assistant cross. You're the fastest, but you'll have to slow down a little bit to match the Assistant's pace. You can't just leave her alone in the dark in the middle of a rickety bridge with zombies on the loose, right?
Trip Two: You = One Minute
Once the Assistant is safe on the other side, you've got to run back across the bridge so the other members of your party have the light they need to make the crossing on their own.
Trip Three: Professor + Janitor = 10 Minutes
The Janitor holds the lantern, with the Professor following behind; however, as was the case when you crossed with the Assistant, the Janitor will have to slow down a little bit to match the Professor's pace. He's old. Give him a break.
Trip Four: Assistant = Two Minutes
After the two slowpokes are safe, the Assistant can take charge of the lantern and run back to you, all alone on the other side with the zombies nipping at your heels.
Trip Five: You + Assistant = Two Minutes
Then you both cross together again.
Total: 17 Minutes
Which gives you just enough time to cut the ropes of the bridge so the zombies can't get at you.
Want to know more? Here's the complete video featuring the full riddle and its solution. Good luck, zombie hunters!
