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John throws a coin until a series of three consecutive heads

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John throws a coin until a series of three consecutive heads [#permalink]

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New post 30 Jun 2007, 08:06
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John throws a coin until a series of three consecutive heads or three consecutive tails appears. What is the probability that the game will end after the fourth throw?

1. 1/16
2. 2/16
3. 3/16
4. 4/16
5. 6/16

Please explain your answer.
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Re: PS Probability [#permalink]

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New post 30 Jun 2007, 08:16
vshaunak@gmail.com wrote:
John throws a coin until a series of three consecutive heads or three consecutive tails appears. What is the probability that the game will end after the fourth throw?

1. 1/16
2. 2/16
3. 3/16
4. 4/16
5. 6/16

Please explain your answer.


B for me.

The probability of getting heads=probability of getting tails=1/2

If the game ends after the fourth throw, there are two possibilities.
T,H,H,H or H,T,T,T.

Therefore, required probability= 2 * [(1/2)^4] = 2/16 (or 1/8 :-D )
Re: PS Probability   [#permalink] 30 Jun 2007, 08:16
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John throws a coin until a series of three consecutive heads

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