TED-Ed/YouTube

This Ted-ED riddle wants you to use maths to escape zombies

Can you solve this riddle and make it across the bridge?

JOSH HRALA
20 NOV 2016
 

Here’s a riddle for you: pretend you’re a grad student who just got an awesome internship in a remote mountain lab where it’s just you, a lab assistant, an old professor, and a janitor.

 

As you explore the complex on your first day, all pumped up that someone is funding your extremely narrow research project, you come across a lever with a skull on it.

Without thinking, you pull it. An alarm immediately sounds, lights flash, and you realise that lever was actually used to contain a horde of angry – and brain-hungry – zombies.

Scared, panicky, and a bit puzzled by the fact that no one had warned you about the weird zombie room when you first arrived, you gather your three companions and make a break for it, running from the lab as the zombies slowly chase after you.

To make matters worse, it’s dark, you have only one lantern, and the only way to safety is across a rickety, old bridge that can only hold two people at once because... why not? 

Being young and spry, you know that you can make it across this deadly bridge in 1 minute. The lab assistant says she can do it in 2 minutes, while the old janitor can manage in 5. The professor, being ancient, needs 10 minutes.

The zombies – the old professor calculates – will take a little over 17 minutes to reach you.

 

Can you figure out how to get everyone across the bridge safely before the zombies show up, remembering that it can only hold two people at a time and you must use the lantern to not slip and fall – Gandalf-style – into the abyss below?

This is the challenge presented in the TED-Ed video above. It’s called the bridge riddle, though there have been different names for similar logic puzzles dating all the way back to the writings of Alcuin of York (735-804 AD).

For this one in particular, you must find a way to usher everyone across the dangerous bridge while sending the lantern back across each time in under 17 minutes.

For example, you could first accompany the old professor, who takes 10 minutes, run back and grab the 2-minute lab assistant, and finish up with the janitor who takes 5 minutes.

But this wouldn’t work, because your total elapsed time would come out to be 19 minutes, making you zombie food or a pile of your former self at the bottom of a pit.

So, what will you do?

We’ll let TED-Ed explain the actual solution, but here’s a hint: it’s all about minimising the time wasted by the two slowest people.

Good luck!

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