The Bridge Riddle

YouTube video

It might not be a good idea to do this internship in a remote mountain lab. It’s also not wise to pull the handle with the skull symbol just to find out what it does. But now is not the time to regret because you have to escape from these mutant zombies quickly. Can you use math to get yourself and your friends across before the zombies arrive? Alex Gendler explains how to do this.


Taking that internship in a remote mountain lab might not have been the best idea. Pulling that lever with the skull symbol to see what it did probably wasn’t so smart, either, but now is not the time for regrets because you need to get away from these mutant zombies fast. With you are the janitor, the lab assistant, and the old professor.

You’ve gained a headstart, but there’s only one way to safety: across an old rope bridge spanning a massive gorge. You can dash across in a minute while the lab assistant takes two minutes. The janitor is a bit slower and needs five minutes, and the professor takes a whole ten minutes, holding onto the ropes every step of the way. By the professor’s calculations, the zombies will catch up to you in just over 17 minutes, so you only have that much time to get everyone across and cut the ropes. Unfortunately, the bridge can only hold two people at a time.

To make matters worse, it’s so dark out that you can barely see, and the old lantern you grabbed on your way only illuminates a tiny area. Can you figure out a way to have everyone escape in time? Remember: no more than two people can cross the bridge together, anyone crossing must either hold the lantern or stay right next to it, and any of you can safely wait in the dark on either side of the gorge.

Most importantly, everyone must be safely across before the zombies arrive. Otherwise, the first zombie could step on the bridge while people are still on it. Finally, there are no tricks to use here. You can’t swing across, use the bridge as a raft, or befriend the zombies. Pause the video now if you want to figure it out for yourself! Answer in: 3 Answer in 2 Answer in 1 At first, it might seem like no matter what you do, you’re just a minute or two short of time, but there is a way.

The key is to minimize the time wasted by the two slowest people by having them cross together. And because you’ll need to make a couple of return trips with the lantern, you’ll want to have the fastest people available to do so. So, you and the lab assistant quickly run across with the lantern, though you have to slow down a bit to match her pace. After two minutes, both of you are across, and you, as the quickest, run back with the lantern. Only three minutes have passed.

So far, so good. Now comes the hard part. The professor and the janitor take the lantern and cross together. This takes them ten minutes since the janitor has to slow down for the old professor, who keeps muttering that he probably shouldn’t have given the zombies night vision. By the time they’re across, there are only four minutes left, and you’re still stuck on the wrong side of the bridge.

But remember, the lab assistant has been waiting on the other side, and she’s the second-fastest of the group. So she grabs the lantern from the professor and runs back across to you. Now with only two minutes left, the two of you make the final crossing. As you step on the far side of the gorge, you cut the ropes and collapse the bridge behind you, just in the nick of time. Maybe next summer, you’ll stick to the library.

Ali Kaya


Ali Kaya

This is Ali. Bespectacled and mustachioed father, math blogger, and soccer player. I also do consult for global math and science startups.

Similar Videos

Binary Counter | Video | Abakcus

Binary Counter

Are you looking for a stunning math project idea to showcase binary numbers? Then, here is a beautiful mechanical binary counter for you! With its intricate design, this counter provides…