Can you solve the stolen rubies riddle? - Dennis Shasha

3,790,538 views ใƒป 2018-10-25

TED-Ed


์•„๋ž˜ ์˜๋ฌธ์ž๋ง‰์„ ๋”๋ธ”ํด๋ฆญํ•˜์‹œ๋ฉด ์˜์ƒ์ด ์žฌ์ƒ๋ฉ๋‹ˆ๋‹ค.

๋ฒˆ์—ญ: lee yun keun ๊ฒ€ํ† : Jihyeon J. Kim
00:07
One of the kingdomโ€™s most prosperous merchants
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์™•๊ตญ์˜ ๊ฐ€์žฅ ๋ฒˆ์ฐฝํ•œ ๋ฌด์—ญ์ƒ์˜
00:09
has been exposed for his corrupt dealings.
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๋ถ€๋‹นํ•œ ๊ฑฐ๋ž˜๊ฐ€ ๋“œ๋Ÿฌ๋‚ฌ์Šต๋‹ˆ๋‹ค.
00:13
Nearly all of his riches
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๋Œ€๋ถ€๋ถ„์˜ ์ž์‚ฐ์„
00:14
are invested in a collection
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์ •๊ตํ•œ ๋ฒ„๋งˆ ๋ฃจ๋น„ 30๊ฐœ ์ˆ˜์ง‘ํ’ˆ์— ํˆฌ์žํ–ˆ๋Š”๋ฐ
00:16
of 30 exquisite Burmese rubies,
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00:19
and the crowd in the square
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๊ด‘์žฅ์˜ ๊ตฐ์ค‘๋“ค์ด
00:21
is clamoring for their confiscation
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ํ”ผํ•ด์ž์˜ ๋ฐฐ์ƒ์„ ์œ„ํ•ด ๊ทธ๊ฒƒ์„ ๋ชฐ์ˆ˜ํ•˜๋ผ๊ณ  ์•„์šฐ์„ฑ ์น˜๊ณ  ์žˆ์Šต๋‹ˆ๋‹ค.
00:23
to reimburse his victims.
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00:25
But the scoundrel and his allies at court
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๊ทธ๋Ÿฌ๋‚˜ ๋ฒ•์ •์— ์žˆ๋˜ ์•…๋‹น๋“ค๊ณผ ๊ทธ์˜ ๊ณต๋ฒ”๋“ค์ด
00:28
have made a convincing case
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์„ค๋“๋ ฅ ์žˆ๋Š” ์ฃผ์žฅ์„ ํ–ˆ๋Š”๋ฐ
00:29
that at least some of his wealth
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์ ์–ด๋„ ๋ถ€์˜ ์ผ๋ถ€๋Š”
00:31
was obtained legitimately,
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ํ•ฉ๋ฒ•์ ์œผ๋กœ ์–ป์€ ๊ฒƒ์ด๋ฉฐ
00:33
and through good service to the crown.
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์™•์—๊ฒŒ ๋ด‰์‚ฌํ•˜์—ฌ ์–ป์€ ๊ฒƒ์ด๋ผ๊ณ  ํ–ˆ์ฃ .
00:35
The king ponders for a minute
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์™•์€ ์ž ์‹œ ๊ณ ๋ฏผํ•˜๊ณ 
00:37
and announces his judgment.
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ํŒ๊ฒฐ์„ ๋ฐœํ‘œํ–ˆ์Šต๋‹ˆ๋‹ค.
00:39
Because thereโ€™s no way to know
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์–ผ๋งˆ๋งŒํผ์˜ ๋ฃจ๋น„๋ฅผ ๋ถ€๋‹นํ•˜๊ฒŒ ์–ป์€ ๊ฒƒ์ธ์ง€ ์•Œ ์ˆ˜๊ฐ€ ์—†๊ธฐ ๋•Œ๋ฌธ์ด์ฃ .
00:40
which portion of the rubies were bought with ill-gotten wealth,
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00:44
the fine will be determined through a game of wits between the merchant
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๋ฒŒ๊ธˆ์€ ๋จธ๋ฆฌ์‹ธ์›€์„ ํ†ตํ•ด ๊ฒฐ์ •๋ ํ…๋ฐ
00:48
and the kingโ€™s most clever advisor โ€“ you.
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์ƒ์ธ๊ณผ ์™•์˜ ๊ฐ€์žฅ ์ถฉ๋ช…ํ•œ ์ฑ…์‚ฌ์ธ ์—ฌ๋Ÿฌ๋ถ„์ด ๊ฒฝ์Ÿํ•˜๋Š” ๊ฒ๋‹ˆ๋‹ค.
00:52
Youโ€™re both told the rules in advance.
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๋‘˜๋‹ค ๊ทœ์น™์„ ๋ฏธ๋ฆฌ ๋“ฃ์Šต๋‹ˆ๋‹ค.
00:54
The merchant will be allowed to discreetly divide his rubies
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์ƒ์ธ์ด ์ƒ์ž ์„ธ ๊ฐœ ์ค‘์—์„œ ์‹ ์ค‘ํ•˜๊ฒŒ ๋ฃจ๋น„๋ฅผ ๋‚˜๋ˆˆ ๋‹ค์Œ
00:57
among three boxes, which will then be placed in front of you.
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์—ฌ๋Ÿฌ๋ถ„ ์•ž์— ๋†“์„ ๊ฒ๋‹ˆ๋‹ค.
01:02
You will be given three cards,
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์—ฌ๋Ÿฌ๋ถ„์€ ์นด๋“œ ์„ธ ์žฅ์„ ๋ฐ›์•„
01:04
and must write a number between 1 and 30 on each,
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๊ฐ ์นด๋“œ์— 1๋ถ€ํ„ฐ 30 ์‚ฌ์ด์˜ ์ˆ˜๋ฅผ ์จ์•ผ ํ•ฉ๋‹ˆ๋‹ค.
01:08
before putting a card in front of each of the boxes.
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๊ฐ ์ƒ์ž ์•ž์— ์นด๋“œ๋ฅผ ๋‘๊ธฐ ์ „์—์š”.
01:12
The boxes will then all be opened.
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๊ทธ๋Ÿฐ ๋‹ค์Œ ๋ชจ๋“  ์ƒ์ž๋ฅผ ์—ด ๊ฒ๋‹ˆ๋‹ค.
01:15
For each box, you will receive exactly
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๊ฐ ์ƒ์ž๋ณ„๋กœ ์—ฌ๋Ÿฌ๋ถ„์€
01:17
as many rubies as the number written on the corresponding card,
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์ ํžŒ ์นด๋“œ์˜ ์ˆ˜๋Œ€๋กœ ๋ฃจ๋น„๋ฅผ ๋ฐ›์„ ๊ฒ๋‹ˆ๋‹ค.
01:21
if the box has that many.
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์ƒ์ž์— ๊ทธ๋งŒํผ ์žˆ๋‹ค๋ฉด์š”.
01:23
But if your number is greater than the number of rubies actually there,
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๊ทธ๋Ÿฌ๋‚˜ ๋งŒ์•ฝ ์ˆซ์ž๊ฐ€ ์‹ค์ œ๋กœ ๊ทธ ์ƒ์ž ์•ˆ์— ์žˆ๋Š” ์ˆ˜๋ณด๋‹ค ํฌ๋ฉด
01:27
the scoundrel gets to keep the entire box.
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์‚ฌ๊ธฐ๊พผ์ด ์ƒ์ž๋ฅผ ์ „๋ถ€ ๊ฐ€์ ธ๊ฐ‘๋‹ˆ๋‹ค.
01:30
The king puts just two constraints on how the scoundrel distributes his rubies.
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์™•์€ ๋ฃจ๋น„๋ฅผ ๋ถ„๋ฐฐํ•˜๋Š”๋ฐ ๋‹จ ๋‘ ๊ฐ€์ง€ ์ œ์•ฝ์„ ๋‘ก๋‹ˆ๋‹ค.
01:36
Each box must contain at least two rubies
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๊ฐ๊ฐ์˜ ์ƒ์ž๋Š” ์ ์–ด๋„ ๋‘ ๊ฐœ์˜ ๋ฃจ๋น„๊ฐ€ ์žˆ์–ด์•ผ ํ•˜๊ณ 
01:40
and one of the boxes must contain exactly six more rubies than anotherโ€”
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์ƒ์ž ํ•˜๋‚˜๋Š” ๋ฐ˜๋“œ์‹œ ๋‹ค๋ฅธ ์ƒ์ž๋ณด๋‹ค ์ •ํ™•ํžˆ 6๊ฐœ๊ฐ€ ๋” ๋งŽ์•„์•ผ ํ•ฉ๋‹ˆ๋‹ค.
01:45
but you wonโ€™t know which boxes those are.
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์—ฌ๋Ÿฌ๋ถ„์€ ์–ด๋Š ์ƒ์ž๊ฐ€ ๊ทธ๋Ÿฐ์ง€๋Š” ๋ชจ๋ฅด์ฃ .
01:48
After a few minutes of deliberation,
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๋ช‡ ๋ถ„ ๋™์•ˆ ๊นŠ์ด ์ƒ๊ฐํ•œ ํ›„์—
01:50
the merchant hides the gems, and the boxes are brought in front of you.
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์ƒ์ธ์€ ๋ณด์„์„ ์ˆจ๊ธฐ๊ณ  ์—ฌ๋Ÿฌ๋ถ„์—๊ฒŒ ์ƒ์ž๋ฅผ ๊ฐ€์ ธ์˜ต๋‹ˆ๋‹ค.
01:54
Which numbers should you choose
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์–ด๋–ค ์ˆซ์ž๋ฅผ ์„ ํƒํ•ด์•ผ
01:56
in order to guarantee the largest possible fine for the scoundrel
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์‚ฌ๊ธฐ๊พผ์—๊ฒŒ ๊ฐ€์žฅ ํฐ ๋ฒŒ๊ธˆ์„ ๋งค๊ธฐ๊ณ 
02:00
and the greatest compensation for his victims?
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ํ”ผํ•ด์ž์—๊ฒŒ๋Š” ๊ฐ€์žฅ ํฐ ๋ณด์ƒ์„ ์ค„ ์ˆ˜ ์žˆ์„๊นŒ์š”?
02:03
Pause the video now if you want to figure it out for yourself.
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๋ฌธ์ œ๋ฅผ ํ’€๊ณ  ์‹ถ์œผ๋ฉด ์˜์ƒ์„ ๋ฉˆ์ถ”์„ธ์š”.
02:07
Answer in 3
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์ •๋‹ต 3์ดˆ ์ „
02:08
Answer in 2
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์ •๋‹ต 2์ดˆ ์ „
02:09
Answer in 1
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์ •๋‹ต 1์ดˆ ์ „
02:11
You donโ€™t want to overshoot by being too greedy.
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๋„ˆ๋ฌด ์š•์‹ฌ๋ถ€๋ ค์„œ ์ˆซ์ž๋ฅผ ํฌ๊ฒŒ ์“ฐ๋ฉด ์•ˆ๋˜๊ฒ ์ฃ .
02:14
But there is a way you can guarantee
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ํ•˜์ง€๋งŒ ์‚ฌ๊ธฐ๊พผ์˜ ๋ณด๋ฌผ์„ ๋ฐ˜ ์ด์ƒ ๊ฐ€์ ธ๊ฐˆ ๋ฐฉ๋ฒ•์ด ์žˆ์Šต๋‹ˆ๋‹ค.
02:16
to get more than half of the scoundrelโ€™s stash.
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02:20
The situation resembles an adversarial game like chess โ€“
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์ƒํ™ฉ์€ ์ฒด์Šค ๊ฐ™์ด ๋Œ€๋ฆฝํ•˜๋Š” ๊ฒŒ์ž„๊ณผ ๋น„์Šทํ•œ๋ฐ
02:24
only here you canโ€™t see the opponentโ€™s position.
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์—ฌ๊ธฐ์„œ๋Š” ๋‹จ์ง€ ์ƒ๋Œ€์˜ ์ƒํ™ฉ์„ ๋ณผ ์ˆ˜ ์—†๋Š” ๊ฒƒ์ด์ฃ .
02:27
To figure out the minimum number of rubies youโ€™re guaranteed to win,
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๊ฐ€์ ธ๊ฐˆ ์ˆ˜ ์žˆ๋Š” ์ตœ์†Œํ•œ์˜ ๋ฃจ๋น„ ์ˆซ์ž๋ฅผ ์•Œ๋ ค๋ฉด
02:31
you need to look for the worst case scenario,
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์ตœ์•…์˜ ์ƒํ™ฉ์„ ์ƒ๊ฐํ•ด์•ผํ•ฉ๋‹ˆ๋‹ค.
02:34
as if the merchant already knew your move
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์ƒ์ธ์ด ์—ฌ๋Ÿฌ๋ถ„์˜ ์ˆ˜๋ฅผ ๋ฏธ๋ฆฌ ์•Œ์•„์„œ
02:36
and could arrange the rubies to minimize your winnings.
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์Šน๋ฅ ์„ ์ตœ์†Œํ™” ์‹œํ‚ค๋„๋ก ๋ฃจ๋น„๋ฅผ ๋ฐฐ์—ดํ•˜๋Š” ๊ฒƒ์ฒ˜๋Ÿผ์š”.
02:40
Because you have no way of knowing which boxes will have more or fewer rubies,
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์–ด๋Š ์ƒ์ž์— ๋ฃจ๋น„๊ฐ€ ๋” ํ˜น์€ ๋œ ๋“ค์–ด์žˆ๋Š”์ง€ ๋ชจ๋ฅด๊ธฐ ๋•Œ๋ฌธ์—
02:44
you should pick the same number for each.
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๊ฐ๊ฐ€ ๊ฐ™์€ ์ˆ˜๋ฅผ ์„ ํƒํ•ด์•ผ ํ•ฉ๋‹ˆ๋‹ค.
02:47
Suppose you write three 9โ€™s.
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9๋ฅผ ์„ธ ๊ฐœ ์ผ๋‹ค๊ณ  ํ•ด๋ด…์‹œ๋‹ค.
02:49
The scoundrel might have allocated the rubies as 8, 14 and 8.
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์‚ฌ๊ธฐ๊พผ์€ 8,14, 8 ์ด๋ ‡๊ฒŒ ๋ฐฐ์—ดํ–ˆ์„ ์ˆ˜ ์žˆ์–ด์š”.
02:54
In that case, youโ€™d receive 9 from the middle box and no others.
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์ด ๊ฒฝ์šฐ ์—ฌ๋Ÿฌ๋ถ„์€ ์ค‘๊ฐ„ ์ƒ์ž์—์„œ 9๊ฐœ๋ฅผ ๋ฐ›๊ณ  ๋‚˜๋จธ์ง€๋Š” ๋ชป ๋ฐ›์ฃ .
02:59
On the other hand, you can be sure that at least two boxes
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๋ฐ˜๋ฉด์— ์ ์–ด๋„ ๋‘ ์ƒ์ž๋Š”
03:02
have a minimum of 8 rubies.
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์ตœ์†Œ 8๊ฐœ๊ฐ€ ์žˆ์Œ์„ ์•Œ ์ˆ˜ ์žˆ์ฃ .
03:04
Hereโ€™s why.
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์ด์œ ๋Š” ์ด๋ ‡์Šต๋‹ˆ๋‹ค.
03:05
Weโ€™ll start by assuming the opposite,
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๋ฐ˜๋Œ€ ๊ฒฝ์šฐ๋ถ€ํ„ฐ ์ƒ๊ฐํ•ด๋ณด์ฃ .
03:08
that two boxes have 7 or fewer.
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์ƒ์ž ๋‘ ๊ฐœ๊ฐ€ 7์ด๋‚˜ ๊ทธ ์ดํ•˜๋ผ๊ณ  ํ•ด๋ด…์‹œ๋‹ค.
03:10
Those could not be the two that differ by 6,
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6๋งŒํผ ์ฐจ์ด๋‚˜๊ฒŒ ๋‘ ์ƒ์ž๊ฐ€ ๊ทธ๋Ÿด ์ˆ˜๋Š” ์—†์ฃ .
03:13
because every box must have at least 2 rubies.
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๋ชจ๋“  ์ƒ์ž๊ฐ€ ์ ์–ด๋„ ๋ฃจ๋น„ 2๊ฐœ๋Š” ์žˆ์–ด์•ผ ํ•˜๋‹ˆ๊นŒ์š”.
03:16
In that case, the third box would have at most 13 rubiesโ€”thatโ€™s 7 plus 6.
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์ด ๊ฒฝ์šฐ ์„ธ ๋ฒˆ์งธ ์ƒ์ž๋Š” 7+6์ธ ์ตœ๋Œ€ 13๊ฐœ๊ฐ€ ์žˆ์„ ๊ฒ๋‹ˆ๋‹ค.
03:23
Add up all three of those boxes,
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์„ธ ๊ฐœ๋ฅผ ๋ชจ๋‘ ํ•ฉํ•ด๋ณด๋ฉด
03:25
and the most that could equal is 27.
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์ตœ๋Œ€ ์ˆ˜๋Š” 27์ด ๋ฉ๋‹ˆ๋‹ค.
03:28
Since thatโ€™s less than 30,
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๊ทธ๊ฑด 30๋ณด๋‹ค ์ ์€ ์ˆ˜์ด๋ฏ€๋กœ
03:30
this scenario isnโ€™t possible.
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์ด๊ฒƒ์€ ๋ถˆ๊ฐ€๋Šฅํ•ฉ๋‹ˆ๋‹ค.
03:32
You now know, by whatโ€™s called a proof by contradiction,
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๋ชจ์ˆœ๋˜๋Š” ์ฆ๊ฑฐ๋ฅผ ํ†ตํ•ด ์•Œ ์ˆ˜ ์žˆ์Šต๋‹ˆ๋‹ค.
03:35
that two of the boxes have 8 or more rubies.
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์ƒ์ž ๋‘ ๊ฐœ๋Š” 8๊ฐœ ๋˜๋Š” ๊ทธ ์ด์ƒ ๋ฃจ๋น„๊ฐ€ ์žˆ์–ด์•ผ ํ•˜์ฃ .
03:39
If you ask for 8 from all three boxes
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์ƒ์ž ์„ธ ๊ฐœ๋ฅผ ๋ชจ๋‘ 8์ด๋ผ๊ณ  ํ•˜๋ฉด
03:42
youโ€™ll receive at least 16โ€”
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์ ์–ด๋„ 16๊ฐœ๋ฅผ ๋ฐ›์„ ๊ฒ๋‹ˆ๋‹ค.
03:44
and thatโ€™s the best you can guarantee,
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๊ทธ๊ฒŒ ์ตœ๋Œ€ํ•œ ๋ณด์žฅํ•  ์ˆ˜ ์žˆ๋Š” ๊ฒƒ์ด์ฃ .
03:46
as you can see by thinking again about the 8, 14, 8 scenario.
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๋‹ค์‹œ ํ•œ ๋ฒˆ 8,14,8์„ ์ƒ๊ฐํ•ด๋ณด๋ฉด์š”.
03:51
Youโ€™ve recovered more than half the scoundrelโ€™s fortune
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์†ํ•ด๋ฐฐ์ƒ์„ ์œ„ํ•ด ์‚ฌ๊ธฐ๊พผ์˜ ์žฌ์‚ฐ ๋ฐ˜ ์ด์ƒ์„ ์–ป๊ฒŒ ๋ฉ๋‹ˆ๋‹ค.
03:54
as restitution for the public.
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03:56
And though heโ€™s managed to hold on to some of his rubies,
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๋ฌผ๋ก  ์ƒ์ธ์ด ๋ฃจ๋น„๋ฅผ ์กฐ๊ธˆ ์ง€ํ‚ค๊ธด ํ–ˆ์ง€๋งŒ
03:59
his fortune has definitely lost some of its shine.
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๋ถ„๋ช… ์žฌ์‚ฐ์ด ๋น›๋ฐ”๋ž˜๊ธด ํ–ˆ์Šต๋‹ˆ๋‹ค.
์ด ์›น์‚ฌ์ดํŠธ ์ •๋ณด

์ด ์‚ฌ์ดํŠธ๋Š” ์˜์–ด ํ•™์Šต์— ์œ ์šฉํ•œ YouTube ๋™์˜์ƒ์„ ์†Œ๊ฐœํ•ฉ๋‹ˆ๋‹ค. ์ „ ์„ธ๊ณ„ ์ตœ๊ณ ์˜ ์„ ์ƒ๋‹˜๋“ค์ด ๊ฐ€๋ฅด์น˜๋Š” ์˜์–ด ์ˆ˜์—…์„ ๋ณด๊ฒŒ ๋  ๊ฒƒ์ž…๋‹ˆ๋‹ค. ๊ฐ ๋™์˜์ƒ ํŽ˜์ด์ง€์— ํ‘œ์‹œ๋˜๋Š” ์˜์–ด ์ž๋ง‰์„ ๋”๋ธ” ํด๋ฆญํ•˜๋ฉด ๊ทธ๊ณณ์—์„œ ๋™์˜์ƒ์ด ์žฌ์ƒ๋ฉ๋‹ˆ๋‹ค. ๋น„๋””์˜ค ์žฌ์ƒ์— ๋งž์ถฐ ์ž๋ง‰์ด ์Šคํฌ๋กค๋ฉ๋‹ˆ๋‹ค. ์˜๊ฒฌ์ด๋‚˜ ์š”์ฒญ์ด ์žˆ๋Š” ๊ฒฝ์šฐ ์ด ๋ฌธ์˜ ์–‘์‹์„ ์‚ฌ์šฉํ•˜์—ฌ ๋ฌธ์˜ํ•˜์‹ญ์‹œ์˜ค.

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