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경우의수/확률/통계amc 2019 b21번 문제

kmb9327
2021-02-13
조회수 150

Debra flips a fair coin repeatedly, keeping track of how many heads and how many tails she has seen in total, until she gets either two heads in a row or two tails in a row, at which point she stops flipping. What is the probability that she gets two heads in a row but she sees a second tail before she sees a second head?

$\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\textbf{(A) } \\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\frac{1}{36} \\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\qquad \\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\textbf{(B) } \\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\frac{1}{24} \\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\qquad \\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\textbf{(C) } \\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\frac{1}{18} \\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\qquad \\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\textbf{(D) } \\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\frac{1}{12} \\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\qquad \\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\textbf{(E) } \\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\frac{1}{6}$

정답은 2번이구요

풀이를 보면 처음이 tht로 시작한다고 하고 ththh, thththh,... 이런 배열이 가능하다고 하고(이 부분은 알겠습니다)

h로는 시작 할 수 없다고 합니다 

그런데 왜 hththh와 같은 배열이 왜 안되는지 이해가 되지 않아서요

답변 주시는 분들께 미리 감사 인사 드립니다

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