Favorite Math Problem
Zombie bridge riddle
Taking that internship in a remote mountain lab might not have been the best idea. Pulling that level with the skull symbol just to see what it did probably wasn't 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 gotten a headstart, but there's only one way to safety: across an old 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 needs a whole ten minutes, holding onto the ropes every step of the way.
By the professors 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.
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.
Can you figure out a way to have everyone escape in time?
With you are the janitor, the lab assistant, and the old professor.
You've gotten a headstart, but there's only one way to safety: across an old 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 needs a whole ten minutes, holding onto the ropes every step of the way.
By the professors 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.
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.
Can you figure out a way to have everyone escape in time?
Answer
At first, it may 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 having the two slowest people cross together.
To start, you and the lab assistant cross together. After two minutes, both of you are across, and you, the quickest, run back to return the lantern. Only three minutes have passed.
You hand the lantern to the janitor and the professor so they can cross. It takes ten minutes. By the time they have crossed there are four minutes remaining.
The lab assistant, being the second fastest, runs back to you with the lantern. With only two minutes remaining, the two of you make the final crossing.
As you make it, you cut the ropes and collapse the bridge behind you just in time.
To start, you and the lab assistant cross together. After two minutes, both of you are across, and you, the quickest, run back to return the lantern. Only three minutes have passed.
You hand the lantern to the janitor and the professor so they can cross. It takes ten minutes. By the time they have crossed there are four minutes remaining.
The lab assistant, being the second fastest, runs back to you with the lantern. With only two minutes remaining, the two of you make the final crossing.
As you make it, you cut the ropes and collapse the bridge behind you just in time.