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Math Help - Probability

  1. #1
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    Probability

    Hi all =)

    Question...
    A fair coin is tossed thrice. Supposed we denote a "head" turning up as 1 and
    "tail" as 0. Given that the total on all three tosses is an odd number,
    what is the probability that at first toss, we get a "head"?

    i dont have any idea to start answering this question..
    please guide me.. =)
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  2. #2
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    Quote Originally Posted by nameck View Post
    Question...
    A fair coin is tossed thrice. Supposed we denote a "head" turning up as 1 and
    "tail" as 0. Given that the total on all three tosses is an odd number,
    what is the probability that at first toss, we get a "head"?
    Here is your outcome set. You count them
    \begin{array}{ccc}<br />
   1 & 1 & 1  \\<br />
   1 & 1 & 0  \\<br />
   1 & 0 & 1  \\<br />
   1 & 0 & 0  \\<br />
   0 & 1 & 1  \\<br />
   0 & 1 & 0  \\<br />
   0 & 0 & 1  \\<br />
   0 & 0 & 0  \\<br /> <br />
 \end{array}
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  3. #3
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    Quote Originally Posted by Plato View Post
    Here is your outcome set. You count them
    \begin{array}{ccc}<br />
   1 & 1 & 1  \\<br />
   1 & 1 & 0  \\<br />
   1 & 0 & 1  \\<br />
   1 & 0 & 0  \\<br />
   0 & 1 & 1  \\<br />
   0 & 1 & 0  \\<br />
   0 & 0 & 1  \\<br />
   0 & 0 & 0  \\<br /> <br />
 \end{array}
    should i use conditional probability?
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  4. #4
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    Quote Originally Posted by nameck View Post
    should i use conditional probability?
    How many odd sums?
    How many odd sums start with a 1?
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  5. #5
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    Quote Originally Posted by Plato View Post
    How many odd sums start with a 1?
    four..
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  6. #6
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    Quote Originally Posted by nameck View Post
    four..
    There are four odd sums. How many start with a one?
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  7. #7
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    There are four odd sums and there are four start with a one.
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  8. #8
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    Quote Originally Posted by nameck View Post
    There are four odd sums and there are four start with a one.
    That is wrong. Count again.
    \begin{array}{ccc}<br />
   1 & 1 & 1  \\<br />
   1 & 0 & 0  \\<br />
   0 & 1 & 0  \\<br />
   0 & 0 & 1  \\<br /> <br />
 \end{array}
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  9. #9
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    1 1 1
    1 1 0
    1 0 1
    1 0 0

    0 1 1
    0 1 0
    0 0 1
    0 0 0



    first four, start with a one.. right?
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  10. #10
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    Quote Originally Posted by nameck View Post
    first four, start with a one.. right?
    No. Of the odd sums how many start with a 1?
    That is the idea of conditional probability.
    The given narrows the outcome space.
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  11. #11
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    i see...
    sorry plato.. i'm kinda slow..
    the answer is two..
    correct?
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