The Bridge Problem
You get hired by a science industry. In the middle of the night, you wake up to get some cookies. While you are downstairs in the laboratory kitchen, you see a button that says, "Do Not Press." Guessed it yet? That's right, you pressed it.
The button opens up a few capsules that were holding zombies, and now they are after you. Quickly, you put your coat on and wake up the others, (who I will introduce in a minute.) When you get outside, you all start heading away from the lab, with the zombies chasing you out. Eventually, you run to a bridge, which is the only way to safety.
The bridge is too rickety that if more that two people at a time even step on it, It will collapse under your feet and you will fall into a huge gorge. Plus, it's so dark outside that the lantern you happened to salvage when evacuating the lab can only illuminate a small area of the bridge. This means that you have to hold the lantern or be right next to the person who is holding it.
Each of you take your own time to run across the bridge:
| You | 1 Minute |
| Assistant | 2 Minutes |
| Janitor | 5 Minutes |
| Elderly Professor | 10 Minutes |
Here, you can see them showing how fast they run, in minutes:
The professor calculates it will take 17 minutes for the zombies to catch up to you.
How many rounds will it take to get everyone across the bridge safely?
The answer is 5.
This section requires Javascript.
You are seeing this because something didn't load right. We suggest you, (a) try
refreshing the page, (b) enabling javascript if it is disabled on your browser and,
finally, (c)
loading the
non-javascript version of this page
. We're sorry about the hassle.
1 solution
It will take 5 rounds. First, you and your assistant take the lantern and pass the bridge. This takes 2 minutes. Then, you, being the quickest, take the lantern and run across the bridge to hand it to the janitor and professor. That is another minute, which totals up to 3 minutes. When the professor and the janitor take the lantern and run across it takes 10 minutes. Now you have used 13 of 17 minutes. But you are still on the wrong side of the bridge. Now remember, your assistant is on the right side of the bridge. She takes the lantern from the janitor and professor and runs to you. 15 minutes have gone past. Then, last, you and the assistant run across once again to total up to 17 minutes, and 5 rounds taken.
Idea from Ted-Ed and Alex Gandler