The coin flip conundrum - Po-Shen Loh

673,070 views ・ 2018-02-15

TED-Ed


μ•„λž˜ μ˜λ¬Έμžλ§‰μ„ λ”λΈ”ν΄λ¦­ν•˜μ‹œλ©΄ μ˜μƒμ΄ μž¬μƒλ©λ‹ˆλ‹€.

λ²ˆμ—­: Yoonkeun Ji κ²€ν† : Jihyeon J. Kim
00:06
When the Wright brothers had to decide
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라이트 ν˜•μ œλŠ” λͺ¨λž˜ μ‚¬κ΅¬μ—μ„œ 이λ₯™ν•  κ·Έλ“€μ˜ 첫 번째
00:08
who would be the first to fly their new airplane
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λΉ„ν–‰κΈ°λ₯Ό λˆ„κ°€ νƒˆμ§€ κ²°μ •ν•  λ•Œ
00:11
off a sand dune, they flipped a coin.
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동전 λ˜μ§€κΈ°λ‘œ κ²°μ •ν–ˆμŠ΅λ‹ˆλ‹€.
00:14
That was fair:
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μ•„μ£Ό κ³΅ν‰ν•œ λ°©λ²•μ΄μ—ˆμ£ .
00:15
we all know there’s an equal chance of getting heads and tails.
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μœ—λ©΄κ³Ό 뒷면이 λ‚˜μ˜¬ ν™•λ₯ μ΄ 각각 5λŒ€ 5둜 μ•„μ£Ό κ³΅ν‰ν•΄μš”.
00:19
But what if they had a more complicated contest?
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ν•˜μ§€λ§Œ 동전을 μ—¬λŸ¬ 번 λ˜μ§€λ©΄ μ–΄λ–»κ²Œ λ κΉŒμš”?
00:22
What if they flippedΒ coinsΒ repeatedly,
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동전을 계속 ν•΄μ„œ λ˜μ§„ λ‹€μŒ
00:24
so that Orville would win as soon as two heads showed up in a rowΒ on his coin,
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동전이 두 번 연속 μœ—λ©΄μΌ λ•Œ 였빌이 이기고
00:29
and Wilbur would win as soon as heads was immediately followed by tailsΒ on his?
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μœ—λ©΄ λ‹€μŒ μ•„λž«λ©΄μ΄ λ‚˜μ™€μ•Ό μœŒλ²„κ°€ 이긴닀면
00:35
Would each brother still have had an equal chance to be the first in flight?
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κ·Έλž˜λ„ 쑰건이 μ—¬μ „νžˆ κ³΅ν‰ν•˜λ‹€κ³  ν•  수 μžˆμ„κΉŒμš”?
00:40
At first, it may seem they’d still have the same chance of winning.
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얼핏 보기엔 곡평해 보일지도 λͺ¨λ¦…λ‹ˆλ‹€.
00:44
There are four combinations for two consecutive flips.
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두 개의 결과에 λ„€ 가지 쑰합이 μ‘΄μž¬ν•˜μ£ .
00:48
And if you do flip a coin just twice,
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λ§Œμ•½ λ‹¨μˆœνžˆ 동전을 두 번 λ˜μ§€κΈ°λ§Œ ν•œλ‹€λ©΄
00:50
there’s an equal chance of each one -- 25%.
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각각의 경우의 μˆ˜κ°€ λ‚˜μ˜¬ ν™•λ₯ μ€ 4λΆ„μ˜ 1이 되겠죠.
00:54
So your intuition might tell you that in any string of coin flips,
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μ—¬λŸ¬λΆ„μ˜ 직관은 μ–΄λ–€ 쑰합을 μ„ νƒν•˜λ“ 
00:58
each combination would have the same shot at appearing first.
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μ²˜μŒμ€ μœ—λ©΄μ΄λ‹ˆ λ˜‘κ°™μ€ νšŸμˆ˜κ°€ μ†Œμš”λ κ±°λΌ μƒκ°ν•˜κ² μ§€λ§Œ
01:02
Unfortunately, you’d be wrong.
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μ•ˆνƒ€κΉκ²Œλ„ ν‹€λ ΈμŠ΅λ‹ˆλ‹€.
01:04
Wilbur actually has a big advantage in this contest.
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μœŒλ²„κ°€ 사싀 훨씬 μœ λ¦¬ν•©λ‹ˆλ‹€.
01:08
Imagine our sequence of coin flips as a sort of board game,
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λ™μ „λ˜μ§€κΈ°λ₯Ό μΌμ’…μ˜ λ³΄λ“œκ²Œμž„μ΄λΌκ³  생각해 λ³΄μ„Έμš”.
01:12
where every flip determines which path we take.
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ν•œ 번 던질 λ•Œλ§ˆλ‹€ μ–΄λ””λ‘œ 이동할 지 μ •ν•  수 μžˆλ‹€κ³  말이죠.
01:15
The goal is to get from start to finish.
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λͺ©ν‘œλŠ” μ‹œμž‘λΆ€ν„° μ’…μ κΉŒμ§€ μ΄λ™ν•˜λŠ” κ²ƒμž…λ‹ˆλ‹€.
01:18
The heads/tails board looks like this.
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μœ—λ©΄-μ•„λž«λ©΄ λ§νŒμ€ μ΄λ ‡κ²Œ μƒκ²ΌμŠ΅λ‹ˆλ‹€.
01:21
And this is the head/head board.
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μœ—λ©΄-μœ—λ©΄ λ§νŒμ€ μ΄λ ‡κ²Œ μƒκ²Όκ³ μš”.
01:24
There’s one critical difference.
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ν•œ 가지 큰 차이점이 μžˆλŠ”λ°μš”.
01:26
Heads/heads has a move that sends you all the way back to the start
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μœ—λ©΄-μœ—λ©΄ λ§νŒμ—μ„œλŠ” μ‹€νŒ¨ν•˜λ©΄ μ‹œμž‘μœΌλ‘œ μ΄λ™ν•©λ‹ˆλ‹€.
01:30
that heads/tails doesn’t have.
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μœ—λ©΄-μ•„λž«λ©΄μ—μ„œλŠ” 그렇지 μ•Šμ£ .
01:33
That’s why heads/heads takes longer on average.
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ν‰κ· μ μœΌλ‘œ μœ—λ©΄-μœ—λ©΄μ΄ 더 λ§Žμ€ μ‹œκ°„μ„ μ†Œμš”ν•˜κ²Œ λ©λ‹ˆλ‹€.
01:36
So we can demonstrate that this is true using probability and algebra
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각각 쑰합에 μ–Όλ§ˆλ‚˜ λ§Žμ€ νšŸμˆ˜κ°€ μ†Œμš”λ˜λŠ”μ§€
01:41
to calculate the average number of flips it would take to get each combination.
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ν™•λ₯ κ³Ό λŒ€μˆ˜ν•™μ„ ν†΅ν•΄μ„œλ„ 증λͺ…ν•  수 μžˆλŠ”λ°μš”.
01:46
Let’s start with the heads/tails board,
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λ¨Όμ € μœ—λ©΄-μ•„λž«λ©΄ 말판으둜 μ‹œμž‘ν•˜μ£ .
01:49
and define x to be the average number of flips to advance one step.
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λ‹€μŒ λ‹¨κ³„λ‘œ λ‚˜κ°€λŠ”λ° κ±Έλ¦° 평균 횟수λ₯Ό X라고 ν•œ λ’€
01:53
Focus only on the arrows.
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ν™”μ‚΄ν‘œλ§Œ 보도둝 ν•˜μ£ .
01:56
It has two identical steps,
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두 가지 λ™μΌν•œ 길이 있고
01:58
each with a 50/50 chance of staying in place or moving forward.
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각각 반반의 ν™•λ₯ λ‘œ μ•žμœΌλ‘œ λ‚˜μ•„κ°€κ±°λ‚˜ κ·Έμžλ¦¬μ— κ·ΈλŒ€λ‘œ μ„œκ²Œ λ©λ‹ˆλ‹€.
02:02
Option 1: If we stay in place by getting tails, we waste one flip.
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μœ—λ©΄-μœ—λ©΄: 뒷면이 λ‚˜μ™€ κ·ΈλŒ€λ‘œ μ„œκ²Œ 되면 ν•œ 번 기회λ₯Ό λ‚ λ¦° 것이 λ©λ‹ˆλ‹€.
02:09
Since we’re back in the same place,
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λ‹€μ‹œ 처음으둜 λŒμ•„κ°€κΈ° λ•Œλ¬Έμ—
02:10
on average we must flip x more times to advance one step.
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X회 더 λ˜μ Έμ•Ό ν•œ 걸음 λ‚˜μ•„κ°ˆ 수 μžˆμŠ΅λ‹ˆλ‹€.
02:15
Together with that first flip,
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처음 μ‹€νŒ¨ν•œ λ˜μ§€κΈ°λ₯Ό ν¬ν•¨ν•˜λ©΄
02:17
this gives an average of x + 1 total flips to advance.
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μ•žμœΌλ‘œ λ‚˜κ°€κΈ° μœ„ν•΄ 평균 x+1νšŒκ°€ μ†Œμš”λ˜λŠ” 꼴이죠.
02:22
Option 2: If we get heads and move forward,
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2단계: μœ—λ©΄μ΄ λ‚˜μ™€ μ•žμœΌλ‘œ μ „μ§„ν–ˆμ„ λ•Œ
02:26
then we have taken exactly one total flip to advance one step.
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μ•žμœΌλ‘œ μ „μ§„ν•˜κΈ° μœ„ν•΄ 1λ²ˆλ°–μ— μ†Œμš”λ˜μ§€ μ•Šμ•˜μŠ΅λ‹ˆλ‹€.
02:30
We can now combine option 1 and option 2 with their probabilities
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1번과 2번의 ν™•λ₯ μ„ μ‘°ν•©ν•˜λ©΄
02:35
to get this expression.
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λ‹€μŒ 곡식을 얻을 수 μžˆμŠ΅λ‹ˆλ‹€.
02:37
Solving that for x gives us an average of two moves to advance one step.
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이 방정식을 ν’€λ©΄ xλŠ” 2κ°€ 되죠.
02:42
Since each step is identical,
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두 단계가 λ™μΌν•˜κΈ° λ•Œλ¬Έμ—
02:44
we can multiply by two and arrive at four flips to advance two steps.
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μœ„μ˜ 값을 λ‹€μ‹œ 2둜 κ³±ν•˜λ©΄ λκΉŒμ§€ μ „μ§„ν•˜κΈ° μœ„ν•΄ ν•„μš”ν•œ νšŸμˆ˜λŠ” 4λ²ˆμž…λ‹ˆλ‹€.
02:50
For heads/heads, the picture isn’t as simple.
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μœ—λ©΄-μœ—λ©΄ λ§νŒμ—μ„œλŠ” 그리 κ°„λ‹¨ν•˜μ§€ μ•Šμ£ .
02:53
This time, let y be the average number of flips to move from start to finish.
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Yκ°€ μ‹œμž‘λΆ€ν„° μ’…μ κΉŒμ§€ κ±Έλ¦¬λŠ” 평균 횟수라고 κ°€μ •ν•˜λ©΄
02:59
There are two options for the first move, each with 50/50 odds.
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첫 번째 μ΄λ™μ—λŠ” 50λŒ€ 50의 두 가지 길이 μžˆμŠ΅λ‹ˆλ‹€.
03:03
Option 1 is the same as before,
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μœ—λ©΄-μœ—λ©΄μ—μ„œλŠ” μ•„κΉŒμ²˜λŸΌ
03:05
getting tails sends us back to the start,
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뒷면이 λ‚˜μ™€μ„œ μ‹œμž‘μœΌλ‘œ λŒμ•„κ°€λŠ” 경우
03:08
giving an average of y+1 total flips to finish.
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λ‹€μŒμœΌλ‘œ μ΄λ™ν•˜κΈ° μœ„ν•΄μ„œλŠ” 평균 y+1번이 μ†Œμš”λ©λ‹ˆλ‹€.
03:12
In Option 2, there are two equally likely cases for the next flip.
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μœ—λ©΄-μ•„λž«λ©΄μ—μ„œλŠ” 두 가지 λ™μΌν•œ
03:17
With heads we’d be done after two flips.
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μœ—λ©΄μ΄ λ‚˜μ˜€λ©΄ 2λ²ˆλ§Œμ— 쒅점에 λ„μ°©ν•˜μ§€λ§Œ
03:20
But tails would return us to the start.
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뒷면이 λ‚˜μ˜€λ©΄ λ‹€μ‹œ μ‹œμž‘μœΌλ‘œ λŒμ•„κ°‘λ‹ˆλ‹€.
03:23
Since we’d return after two flips,
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두 번 λ˜μ§„ 이후 λŒμ•„μ™”μœΌλ―€λ‘œ
03:25
we’d then need an average of y+2 flips in total to finish.
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μ’…μ κΉŒμ§€ κ°€λ €λ©΄ y+2번 걸리게 λ©λ‹ˆλ‹€.
03:31
So our full expression will be this.
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μ’…ν•©ν•˜μžλ©΄ λ‹€μŒκ³Ό κ°™μŠ΅λ‹ˆλ‹€.
03:35
And solving this equation gives us six flips.
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이 방정식을 ν’€λ©΄ 닡은 6이 되죠.
03:38
So the math calculates that it takes an average of six flips to get heads/heads,
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μˆ˜ν•™μ μœΌλ‘œ μœ—λ©΄-μœ—λ©΄μ΄ λ‚˜μ˜€λ €λ©΄ 6λ²ˆμ„ λ˜μ Έμ•Ό ν•˜κ³ 
03:43
and an average of four to get heads/tails.
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μœ—λ©΄-μ•„λž«λ©΄μ€ 4λ²ˆμž…λ‹ˆλ‹€.
03:46
And, in fact, that’s what you’d see if you tested it for yourself enough times.
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μ—¬λŸ¬λΆ„μ΄ μΆ©λΆ„ν•œ 횟수둜 μ‹œν—˜ν•΄λ³΄λ©΄ μœ„μ™€ 같은 κ²°κ³Όλ₯Ό μ–»κ²Œ 될 κ²λ‹ˆλ‹€.
03:52
Of course, the Wright brothers didn’t need to work all this out;
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λ¬Όλ‘  라이트 ν˜•μ œκ°€ μ΄λ ‡κ²Œ λ³΅μž‘ν•œ 과정을 κ±°μΉ˜μ§„ μ•Šμ•˜μ£ .
03:55
they only flipped the coin once, and Wilbur won.
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동전은 ν•œ 번만 던쑌고, μœŒλ²„κ°€ μ΄κ²ΌμŠ΅λ‹ˆλ‹€.
03:59
But it didn’t matter: Wilbur’s flight failed,
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ν•˜μ§€λ§Œ λˆ„κ°€ μ΄κ²ΌλŠ”μ§€λŠ” μ€‘μš”ν•˜μ§€ μ•Šμ•˜μ£ , μœŒλ²„μ˜ 비행은 μ‹€νŒ¨ν–ˆκ³ 
04:02
and Orville made aviation history, instead.
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였빌이 항곡 역사에 이름을 남겼죠.
04:05
Tough luck, Wilbur.
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운이 μ—†μ—ˆλ„€μš”, μœŒλ²„.
이 μ›Ήμ‚¬μ΄νŠΈ 정보

이 μ‚¬μ΄νŠΈλŠ” μ˜μ–΄ ν•™μŠ΅μ— μœ μš©ν•œ YouTube λ™μ˜μƒμ„ μ†Œκ°œν•©λ‹ˆλ‹€. μ „ 세계 졜고의 μ„ μƒλ‹˜λ“€μ΄ κ°€λ₯΄μΉ˜λŠ” μ˜μ–΄ μˆ˜μ—…μ„ 보게 될 κ²ƒμž…λ‹ˆλ‹€. 각 λ™μ˜μƒ νŽ˜μ΄μ§€μ— ν‘œμ‹œλ˜λŠ” μ˜μ–΄ μžλ§‰μ„ 더블 ν΄λ¦­ν•˜λ©΄ κ·Έκ³³μ—μ„œ λ™μ˜μƒμ΄ μž¬μƒλ©λ‹ˆλ‹€. λΉ„λ””μ˜€ μž¬μƒμ— 맞좰 μžλ§‰μ΄ μŠ€ν¬λ‘€λ©λ‹ˆλ‹€. μ˜κ²¬μ΄λ‚˜ μš”μ²­μ΄ μžˆλŠ” 경우 이 문의 양식을 μ‚¬μš©ν•˜μ—¬ λ¬Έμ˜ν•˜μ‹­μ‹œμ˜€.

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