Can you solve the seven planets riddle? - Edwin F. Meyer

4,474,642 views ใƒป 2018-02-26

TED-Ed


ืื ื ืœื—ืฅ ืคืขืžื™ื™ื ืขืœ ื”ื›ืชื•ื‘ื™ื•ืช ื‘ืื ื’ืœื™ืช ืœืžื˜ื” ื›ื“ื™ ืœื”ืคืขื™ืœ ืืช ื”ืกืจื˜ื•ืŸ.

ืชืจื’ื•ื: Ido Dekkers ืขืจื™ื›ื”: Shai Hakim
00:11
Your interstellar police squad has tracked a group of dangerous rebels
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ื”ืžืฉื˜ืจื” ื”ื‘ื™ืŸ ื›ื•ื›ื‘ื™ืช ืฉืœื›ื ืขืงื‘ื” ืื—ืจื™ ืงื‘ื•ืฆื” ืฉืœ ืžื•ืจื“ื™ื ืžืกื•ื›ื ื™ื
00:16
to a cluster of of seven small planets.
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ืœืžืงื‘ืฅ ืฉืœ ืฉื‘ืขื” ื›ื•ื›ื‘ื™ ืœื›ืช.
00:19
Now you must apprehend them quickly before their reinforcements arrive.
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ืขื›ืฉื™ื• ืืช ืฆืจื™ื›ื™ื ืœืชืคื•ืฉ ืื•ืชื ื‘ืžื”ื™ืจื•ืช ืœืคื ื™ ืฉื”ืชื’ื‘ื•ืจืช ืฉืœื”ื ืชื’ื™ืข.
00:24
Of course, the rebels wonโ€™t just stay put.
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ื›ืžื•ื‘ืŸ, ื”ืžื•ืจื“ื™ื ืœื ื™ืฉืืจื• ื‘ืžืงื•ื.
00:27
Theyโ€™ll try to dodge you by moving from planet to planet.
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ื”ื ื™ื ืกื• ืœื—ืžื•ืง ืžื›ื ืขืœ ื™ื“ื™ ืžืขื‘ืจ ืžืคืœื ื˜ื” ืœืคืœื ื˜ื”.
00:31
But you have one major advantage.
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ืื‘ืœ ื™ืฉ ืœื›ื ื™ืชืจื•ืŸ ืื—ื“ ื’ื“ื•ืœ.
00:34
Every hour, your state-of-the-art cruiser can warp between any two planets,
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ื›ืœ ืฉืขื”, ื”ื—ืœืœื™ืช ื”ืžืฉื•ื›ืœืœืช ืฉืœื›ื ื™ื›ื•ืœื” ืœืงืคื•ืฅ ื‘ื™ืŸ ื›ืœ ืฉืชื™ ืคืœื ื˜ื•ืช,
00:40
while their beat-up smuggling ship
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ื‘ื–ืžืŸ ืฉื”ืกืคื™ื ื” ื”ื—ื‘ื•ื˜ื” ืฉืœื”ื
00:42
can only jump to an adjacent planet in that same time.
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ื™ื›ื•ืœื” ืœืงืคื•ืฅ ืจืง ืœื›ื•ื›ื‘ ืฆืžื•ื“ ื‘ืื•ืชื• ื–ืžืŸ.
00:46
These rebels donโ€™t like to stay put.
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ื”ืžื•ืจื“ื™ื ื”ืืœื” ืœื ืื•ื”ื‘ื™ื ืœื”ืฉืืจ ื‘ืžืงื•ื.
00:48
Every time they can relocate, they will.
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ื›ืœ ืคืขื ืฉื”ื ื™ื›ื•ืœื™ื ืœืขื‘ื•ืจ, ื”ื ืขื•ืฉื™ื ื–ืืช.
00:52
Your scouts tell you that the approaching rebel fleet is 10 hours away.
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ื”ืกื™ื™ืจ ืฉืœื›ื ืื•ืžืจ ืœื›ื ืฉืฆื™ ื”ืžื•ืจื“ื™ื ื”ืžืชืงืจื‘ ื‘ืžืจื—ืง ืฉืœ ื‘ืขืจืš 10 ืฉืขื•ืช.
00:57
You canโ€™t risk letting the rebels escape.
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ืืชื ืœื ื™ื›ื•ืœื™ื ืœื”ืกืชื›ืŸ ื‘ื‘ืจื™ื—ืช ื”ืžื•ืจื“ื™ื.
01:00
Can you devise a sequence for searching the planets
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ื”ืื ืืชื ื™ื›ื•ืœื™ื ืœื”ืขืœื•ืช ื“ืจืš ืœื—ื™ืคื•ืฉ ื‘ืคืœื ื˜ื•ืช
01:03
thatโ€™s guaranteed to catch them in 10 warps or less,
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ืฉืžื‘ื˜ื™ื—ื” ืืช ืชืคื™ืฉืชื ื‘ 10 ืงืคื™ืฆื•ืช ืื• ืคื—ื•ืช,
01:07
no matter what moves they make?
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ืœื ืžืฉื ื” ื›ืžื” ืงืคื™ืฆื•ืช ื”ื ื™ืขืฉื•?
01:12
Rounding up the rebels wonโ€™t be easy.
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ืชืคื™ืฉืช ื”ืžื•ืจื“ื™ื ืœื ืชื”ื™ื” ืงืœื”.
01:14
For one, you have no way of knowing which planet theyโ€™re on to begin with.
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ืจืืฉื™ืช, ืื™ืŸ ืœื›ื ื“ืจืš ืœื“ืขืช ืขืœ ืื™ื–ื” ืคืœื ื˜ื” ื”ื ื”ืชื—ื™ืœื•.
01:19
And without that information, itโ€™s hard to determine where theyโ€™ll move next.
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ื•ื‘ืœื™ ื”ืžื™ื“ืข ื”ื–ื”, ื–ื” ืงืฉื” ืœืงื‘ื•ืข ืœืืŸ ื”ื ื™ื ื•ืขื• ื”ืœืื”.
01:25
So where do you begin?
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ืื– ืื™ืคื” ืืชื ืžืชื—ื™ืœื™ื?
01:27
When tackling problems of this kind it often helps to simplify things,
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ื›ืฉืžืชืžื•ื“ื“ื™ื ืขื ื‘ืขื™ื•ืช ืžืกื•ื’ ื–ื”, ื–ื” ืขื•ื–ืจ ืœืจื•ื‘ ืœืคืฉื˜ ืืช ื”ื“ื‘ืจื™ื,
01:31
so you can better understand their dynamics.
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ื›ืš ืฉืชื•ื›ืœื• ืœื”ื‘ื™ืŸ ื˜ื•ื‘ ื™ื•ืชืจ ืืช ื”ื“ื™ื ืžื™ืงื” ืฉืœื”ื.
01:34
Letโ€™s imagine that this cluster has the same arrangement
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ื‘ื•ืื• ื ื“ืžื™ื™ืŸ ืฉืœืžืงื‘ืฅ ื”ื–ื” ื™ืฉ ืื•ืชื• ืกื™ื“ื•ืจ
01:37
but no outermost planets, leaving only the four in the center.
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ืื‘ืœ ื‘ืœื™ ื›ื•ื›ื‘ื™ ืœื›ืช ื—ื™ืฆื•ื ื™ื™ื ืžื” ืฉืžืฉืื™ืจ ืจืง ืืจื‘ืขื” ื‘ืžืจื›ื–.
01:42
We still donโ€™t know which planet the rebels start on.
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ืื ื—ื ื• ืขื“ื™ื™ืŸ ืœื ื™ื•ื“ืขื™ื ืžืื™ื–ื” ืคืœื ื˜ื” ื”ืžื•ืจื“ื™ื ื”ื—ืœื•.
01:45
But thereโ€™s one key feature:
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ืื‘ืœ ื™ืฉ ืชื›ื•ื ืช ืžืคืชื— ืื—ืช:
01:47
the third planet is adjacent to all others,
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ื”ืคืœื ื˜ื” ื”ืฉืœื™ืฉื™ืช ืกืžื•ื›ื” ืœื›ืœ ื”ืื—ืจื•ืช,
01:51
which means the rebels either start there and move somewhere else,
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ืžื” ืฉืื•ืžืจ ืฉื”ืžื•ืจื“ื™ื ืื• ื”ืชื—ื™ืœื• ืฉื ื•ื ืขื• ืœืžืงื•ื ืื—ื“,
01:55
or start on one of the other planets
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ืื• ืฉื”ืชื—ื™ืœื• ื‘ืื—ืช ื”ืคืœื ื˜ื•ืช ื”ืื—ืจื•ืช
01:57
and have no choice but to move to planet three.
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ื•ืื™ืŸ ืœื”ื ื‘ืจื™ืจื” ืืœื ืœื ื•ืข ืœืคืœื ื˜ื” ืฉืœื•ืฉ.
02:02
Simply checking planet three twice in a row would do the trick.
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ืคืฉื•ื˜ ืœื‘ื“ื•ืง ืืช ืคืœื ื˜ื” ืฉืœื•ืฉ ืคืขืžื™ื™ื ื‘ืจืฆืฃ ื™ืคืชื•ืจ ืืช ื”ื‘ืขื™ื”.
02:09
Adding the three outer planet adds a bit more complexity,
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ื”ื•ืกืคืช ืฉืœื•ืฉ ืคืœื ื˜ื•ืช ื—ื™ืฆื•ื ื™ื•ืช ืžื•ืกื™ืคื” ืžืขื˜ ืžื•ืจื›ื‘ื•ืช,
02:13
but the same strategy remains.
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ืื‘ืœ ืื•ืชื” ืืกื˜ืจื˜ื’ื™ื” ื ืฉืืจืช.
02:15
We want to check the planets in an order that will eventually corner the rebels.
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ืื ื—ื ื• ืจื•ืฆื™ื ืœื‘ื“ื•ืง ืืช ื”ืคืœื ื˜ื•ืช ืœืคื™ ืกื“ืจ ืฉื‘ืกื•ืคื• ืฉืœ ื“ื‘ืจ ื™ื“ื—ื•ืฃ ืืช ื”ืžื•ืจื“ื™ื ืœืคื™ื ื”.
02:21
And thereโ€™s another insight that can help us:
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ื•ื™ืฉ ืชื•ื‘ื ื” ื ื•ืกืคืช ืฉื™ื›ื•ืœื” ืœืขื–ื•ืจ ืœื ื•:
02:23
each hour, the rebels move from an even-numbered planet
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ื›ืœ ืฉืขื”, ื”ืžื•ืจื“ื™ื ื ืขื™ื ืžืคืœื˜ื” ืขื ืžืกืคืจ ื–ื•ื’ื™
02:26
to an odd-numbered planet,
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ืœืคืœื˜ื ื” ืขื ืžืกืคืจ ืื™ ื–ื•ื’ื™,
02:28
or vice versa.
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ืื• ืœื”ืคืš.
02:30
This gives us a way to simplify the problem
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ื–ื” ื ื•ืชืŸ ืœื ื• ื“ืจืš ืœืคืฉื˜ ืืช ื”ื‘ืขื™ื”
02:32
by dividing the planets into two subsets,
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ืขืœ ื™ื“ื™ ื—ืœื•ืงืช ื”ืคืœื ื˜ื•ืช ืœืฉืชื™ ืชืชื™ ืงื‘ื•ืฆื•ืช,
02:35
and tackling each one separately.
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ื•ืœื‘ื“ื•ืง ื›ืœ ืื—ืช ื‘ื ืคืจื“.
02:38
For starters, letโ€™s assume the rebels begin on an even-numbered planet:
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ื‘ื”ืชื—ืœื”, ื‘ื•ืื• ื ื ื™ื— ืฉื”ืžื•ืจื“ื™ื ืžืชื—ื™ืœื™ื ืขืœ ืคืœื ื˜ื” ื–ื•ื’ื™ืช:
02:42
either two,
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ืื• ืฉืชื™ื™ื,
02:43
four,
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02:43
or six.
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ืืจื‘ืข,
ืื• ืฉืฉ.
02:45
So weโ€™ll search planet two first.
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ืื– ื ื—ืคืฉ ื‘ืคืœื ื˜ื” ืฉืชื™ื™ื ืงื•ื“ื.
02:47
If theyโ€™re not there, they must have started on either four
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ืื ื”ื ืœื ืฉื, ื”ื ื—ื™ื™ื‘ื™ื ื”ื™ื• ืœื”ืชื—ื™ืœ ืื• ื‘ืืจื‘ืข
02:50
or six.
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ืื• ื‘ืฉืฉ.
02:51
which means they can move to three,
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ืžื” ืฉืื•ืžืจ ืฉื”ื ื™ื›ื•ืœื™ื ืœื ื•ืข ืœืฉืœื•ืฉ,
02:53
five,
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ื—ืžืฉ,
02:54
or seven.
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ืื• ืฉื‘ืข.
02:56
Planet three at the center gives them the most options for their next move,
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ืคืœื ื˜ื” ืฉืœื•ืฉ ื‘ืžืจื›ื– ื ื•ืชื ืช ืœื”ื ืืช ื”ื›ื™ ื”ืจื‘ื” ืืคืฉืจื•ื™ื•ืช ืœืฆืขื“ ื”ื‘ื,
03:00
so weโ€™ll want to check there next.
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ืื– ื ืจืฆื” ืœื‘ื“ื•ืง ืฉื ืจืืฉื™ืช.
03:02
If we donโ€™t find them, they must have been at planet five
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ืื ืื ื—ื ื• ืœื ืžื•ืฆืื™ื ืื•ืชื, ื”ื ื”ื™ื• ื—ื™ื™ื‘ื™ื ืœื”ื™ื•ืช ืขืœ ืคืœื ื˜ื” ื—ืžืฉ
03:05
or seven,
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ืื• ืฉื‘ืข,
03:06
meaning theyโ€™ll next move to four
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ืžื” ืฉืื•ืžืจ ืฉื”ื ื™ืขื‘ืจื• ืœืืจื‘ืข
03:09
or six.
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ืื• ืฉืฉ.
03:10
Letโ€™s now search planet four.
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ื‘ื•ื ื ืขื‘ื•ืจ ืขื›ืฉื™ื• ืœืืจื‘ืข,
03:12
If theyโ€™re not there, they must have gone to the sixth planet
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ืื ื”ื ืœื ืฉื, ื”ื ื—ื™ื™ื‘ื™ื ื”ื™ื• ืœืขื‘ื•ืจ ืœืคืœื ื˜ื” ื”ืฉื™ืฉื™ืช
03:15
and can only flee to three
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ื•ื™ื›ื•ืœื™ื ืœื‘ืจื•ื— ืจืง ืœืฉืœื•ืฉ
03:18
or seven.
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ืื• ืฉื‘ืข.
03:20
If we next scour planet three and donโ€™t find them,
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ืื ื ืกืงื•ืจ ืื– ืืช ืฉืœื•ืฉ ื•ืœื ื ืžืฆื ืื•ืชื,
03:23
we know they went to planet seven and are now cornered.
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ืื ื—ื ื• ื™ื•ื“ืขื™ื ืฉื”ื ืขื‘ืจื• ืœืคืœื ื˜ื” ืฉื‘ืข ื•ืขื›ืฉื™ื• ืžื›ื•ืชืจื™ื.
03:26
They can only move to planet six, where weโ€™ll apprehend them on our fifth search.
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ื”ื ื™ื›ื•ืœื™ื ืœื ื•ืข ืจืง ืœืคืœื ื˜ื” ืฉืฉ, ืฉื ื ืœื›ื•ื“ ืื•ืชื ื‘ื—ื™ืคื•ืฉ ื”ื—ืžื™ืฉื™.
03:32
Of course, this plan only works
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ื›ืžื•ื‘ืŸ, ื”ืชื•ื›ื ื™ืช ืขื•ื‘ื“ืช ืจืง
03:34
assuming that the rebels were on an even-numbered planet in the first hour.
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ื‘ื”ื ื—ื” ืฉื”ืžื•ืจื“ื™ื ื”ื™ื• ืขืœ ื›ื•ื›ื‘ ืœื›ืช ื–ื•ื’ื™ ื‘ื”ืชื—ืœื”.
03:39
But what if that assumption was wrong?
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ืื‘ืœ ืžื” ืื ื”ื”ื ื—ื” ื”ื–ื• ื”ื™ืชื” ืฉื’ื•ื™ื”?
03:41
In that case, they mustโ€™ve started on an odd-numbered planet.
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ื‘ืžืงืจื” ื”ื–ื”, ื”ื ื”ื™ื• ื—ื™ื™ื‘ื™ื ืœื”ืชื—ื™ืœ ืขืœ ืคืœื ื˜ื” ืื™ ื–ื•ื’ื™ืช.
03:44
And because they move to an adjacent planet every hour,
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ื•ื‘ื’ืœืœ ืฉื”ื ื ืขื™ื ืœืคืœื ื˜ื” ืกืžื•ื›ื” ื›ืœ ืฉืขื”.
03:47
their location must alternate between odd and even-numbered planets.
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ื”ืžื™ืงื•ื ืฉืœื”ื ื—ื™ื™ื‘ ืœื”ืฉืชื ื•ืช ื‘ื™ืŸ ืคืœื ื˜ื” ื–ื•ื’ื™ืช ืœืื™ ื–ื•ื’ื™ืช.
03:52
This means that if they were on an odd-numbered planet to start,
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ื–ื” ืื•ืžืจ ืฉืื ื”ื ื”ื™ื• ืขื ืคืœื ื˜ื” ื–ื•ื’ื™ืช ื‘ื”ืชื—ืœื”,
03:55
after five moves, they'd be on an even-numbered planet.
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ืื—ืจื™ ื—ืžื™ืฉื” ืžื”ืœื›ื™ื, ื”ื ื™ื”ื™ื• ืขืœ ืคืœื ื˜ื” ืื™ ื–ื•ื’ื™ืช.
04:00
So if our first five searches missed them
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ืื– ืื ื—ืžืฉืช ื”ื ืกื™ื•ื ื•ืช ื”ืจืืฉื•ื ื™ื ืฉืœื ื• ืคื™ืกืคืกื• ืื•ืชื
04:02
because our assumption that they started on an even-numbered planet was wrong,
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ื‘ื’ืœืœ ืฉื”ื”ื ื—ื” ืฉื”ื ื”ืชื—ื™ืœื• ืขืœ ืคืœื ื˜ื” ื–ื•ื’ื™ืช ื”ื™ืชื” ืฉื’ื•ื™ื”,
04:06
all we have to do now is repeat the sequence!
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ื›ืœ ืžื” ืฉืื ื—ื ื• ืฆืจื™ื›ื™ื ืœืขืฉื•ืช ื–ื” ืœื—ื–ื•ืจ ืขืœ ื”ืจืฆืฃ!
04:09
Searching the planets in order
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ื—ื™ืคื•ืฉ ื‘ืคืœื ื˜ื•ืช ืœืคื™ ื”ืกื“ืจ
04:11
two,
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ืฉืชื™ื™ื,
04:12
three,
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ืฉืœื•ืฉ,
04:13
four,
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ืืจื‘ืข,
04:14
three,
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ืฉืœื•ืฉ,
04:16
six,
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ืฉืฉ,
04:17
two,
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ืฉืชื™ื™ื,
04:18
three,
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ืฉืœื•ืฉ,
04:19
four,
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ืืจื‘ืข,
04:21
three,
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ืฉืœื•ืฉ,
04:22
six,
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ืฉืฉ,
04:23
leaves the rebels nowhere to run.
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ืœื ืžืฉืื™ืจ ืœืžื•ืจื“ื™ื ืœืืŸ ืœื‘ืจื•ื—.
04:25
Thanks to your deductive reasoning, order is restored to the galaxy.
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ื”ื•ื“ื•ืช ืœื”ื’ื™ื•ืŸ ื”ื”ืกืงืชื™ ืฉืœื›ื, ื”ืกื“ืจ ื—ื–ืจ ืœื’ืœืงืกื™ื”.
ืขืœ ืืชืจ ื–ื”

ืืชืจ ื–ื” ื™ืฆื™ื’ ื‘ืคื ื™ื›ื ืกืจื˜ื•ื ื™ YouTube ื”ืžื•ืขื™ืœื™ื ืœืœื™ืžื•ื“ ืื ื’ืœื™ืช. ืชื•ื›ืœื• ืœืจืื•ืช ืฉื™ืขื•ืจื™ ืื ื’ืœื™ืช ื”ืžื•ืขื‘ืจื™ื ืขืœ ื™ื“ื™ ืžื•ืจื™ื ืžื”ืฉื•ืจื” ื”ืจืืฉื•ื ื” ืžืจื—ื‘ื™ ื”ืขื•ืœื. ืœื—ืฅ ืคืขืžื™ื™ื ืขืœ ื”ื›ืชื•ื‘ื™ื•ืช ื‘ืื ื’ืœื™ืช ื”ืžื•ืฆื’ื•ืช ื‘ื›ืœ ื“ืฃ ื•ื™ื“ืื• ื›ื“ื™ ืœื”ืคืขื™ืœ ืืช ื”ืกืจื˜ื•ืŸ ืžืฉื. ื”ื›ืชื•ื‘ื™ื•ืช ื’ื•ืœืœื•ืช ื‘ืกื ื›ืจื•ืŸ ืขื ื”ืคืขืœืช ื”ื•ื•ื™ื“ืื•. ืื ื™ืฉ ืœืš ื”ืขืจื•ืช ืื• ื‘ืงืฉื•ืช, ืื ื ืฆื•ืจ ืื™ืชื ื• ืงืฉืจ ื‘ืืžืฆืขื•ืช ื˜ื•ืคืก ื™ืฆื™ืจืช ืงืฉืจ ื–ื”.

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