Can you solve the bridge riddle? – Alex Gendler

August 16, 2019

Taking that internship
in a remote mountain lab might not have been the best idea. Pulling that lever with the skull symbol
just 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 gotten 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 just stick to the library.

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