Can you solve the jail break riddle? - Dan Finkel
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0:07 - 0:09Your timing made you and your partner
-
0:09 - 0:12the most infamous bank robbers
in the west. -
0:12 - 0:16Now, you’ll need to use that timing
to help you break out of jail. -
0:16 - 0:21At the appointed time, you’ll be walking
in the yard near the electric fence. -
0:21 - 0:22Your partner will flash you the signal,
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0:22 - 0:25and exactly 45 seconds later,
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0:25 - 0:28short out the fence circuit.
-
0:28 - 0:31It’ll automatically restart
after a second or two, -
0:31 - 0:33but as long as you move fast,
-
0:33 - 0:34you’ll be home free.
-
0:34 - 0:36And then you notice,
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0:36 - 0:37to your horror,
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0:37 - 0:38that your watch is broken,
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0:38 - 0:40and there’s no time to fix it.
-
0:40 - 0:41The signal is coming,
-
0:41 - 0:46and if you make even a small mistake
in counting off 45 seconds, -
0:46 - 0:48you’ll get fried.
-
0:48 - 0:49Searching your pockets,
-
0:49 - 0:51you find something that might help:
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0:51 - 0:56a lighter and two fuses you made earlier
in the prison work program. -
0:56 - 0:59Each fuse is a length of flammable twine,
-
0:59 - 1:04built to be lit on either end
and burn for precisely one minute. -
1:04 - 1:08The problem is that even though
the fuses look uniform, -
1:08 - 1:09they don’t burn evenly,
-
1:09 - 1:12so if you cut one in half, for example,
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1:12 - 1:15one side might burn longer than the other.
-
1:15 - 1:18Your partner is going to give
the signal any minute, -
1:18 - 1:19and you’ll have to make your move.
-
1:19 - 1:25How can you use the fuses and lighter
to time exactly 45 seconds? -
1:25 - 1:30Pause the video to figure it out yourself.
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1:30 - 1:32Answer in 3
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1:32 - 1:34Answer in 2
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1:34 - 1:35Answer in 1
-
1:35 - 1:38The length of the fuse may
not tell you anything, -
1:38 - 1:43but you do know the fuses take exactly
60 seconds to burn from end to end. -
1:43 - 1:44Here’s the key insight:
-
1:44 - 1:49If you start a fuse on one side
and it burns for 30 seconds, -
1:49 - 1:52there’ll still be 30 seconds of fuse left.
-
1:52 - 1:54If you had started it from the other end,
-
1:54 - 1:58it would’ve reached the exact
same spot in thirty seconds. -
1:58 - 2:02That means that if you lit it
from both ends simultaneously, -
2:02 - 2:05it would burn out in precisely 30 seconds.
-
2:05 - 2:08But how will you time the last fifteen?
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2:08 - 2:11That’ll have to come from the second fuse.
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2:11 - 2:13If it were a 30 second fuse,
-
2:13 - 2:15you’d be able to use that same trick again
-
2:15 - 2:20to double the burning speed and
make it last exactly 15 seconds. -
2:20 - 2:23And, you realize, you can
shorten the second fuse -
2:23 - 2:27by lighting one end of it at the same time
as you light the first. -
2:27 - 2:29At the moment the first burns out,
-
2:29 - 2:33you’ll be left with 30 seconds
on the second fuse. -
2:33 - 2:35Just when you’ve got this all figured out,
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2:35 - 2:37you see the signal from your partner,
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2:37 - 2:39and spring into action.
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2:39 - 2:43You gather the four ends of the two
fuses and light three of them. -
2:43 - 2:45The moment the first burns out,
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2:45 - 2:48you light the other end
of the second fuse. -
2:48 - 2:49When it flickers and dies,
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2:49 - 2:53you know that exactly 45
seconds have passed, -
2:53 - 2:56and the electric fence is dead.
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2:55 - 2:58By the time it hiccups back to life,
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2:58 - 3:01you’re over the fence and home free.
- Title:
- Can you solve the jail break riddle? - Dan Finkel
- Speaker:
- Daniel Finkel
- Description:
-
View full lesson: https://ed.ted.com/lessons/can-you-solve-the-jail-break-riddle-dan-finkel
Your timing made you and your partner infamous bank robbers. Now, you need to use that timing to break out of jail. Your partner is about to flash you a signal, and exactly 45 seconds later the electric fence circuit will short out. Then you notice that your watch is broken, and all you have are two 1-minute fuses in your pocket. Can you time your escape and get free? Dan Finkel shows how.
Lesson by Dan Finkel, directed by Artrake Studio.
- Video Language:
- English
- Team:
- closed TED
- Project:
- TED-Ed
- Duration:
- 03:01
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