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Math Help - roll a die

  1. #1
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    roll a die

    Help! If I roll a die 8 times...I'd like to know the probability of getting an odd number.

    a) exactly 3 times.
    b) at least 7 times.
    c) at most 6 times.

    using the formula: nCr (p)^r (1-p)^n-r (I couln't figure out how to superscript the appropriate letters. I'm hoping whoever can help me out will know what I'm talking about)
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  2. #2
    Junior Member roy_zhang's Avatar
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    Quote Originally Posted by Pookie View Post
    Help! If I roll a die 8 times...I'd like to know the probability of getting an odd number.

    a) exactly 3 times.
    b) at least 7 times.
    c) at most 6 times.

    using the formula: nCr (p)^r (1-p)^n-r (I couln't figure out how to superscript the appropriate letters. I'm hoping whoever can help me out will know what I'm talking about)
    You are trying to apply the Binomial probability formula here, realize that your n=8 and the probability to get an odd number is p(\mathtt{odd})=\frac{1}{2}. Now you can apply the formula here.

    (a) r=3, so P=\binom{8}{3}\left(\frac{1}{2}\right)^3\left(\fra  c{1}{2}\right)^5

    (b) r=7\;\mathtt{or}\;8, so P=\binom{8}{7}\left(\frac{1}{2}\right)^7\left(\fra  c{1}{2}\right)+\binom{8}{8}\left(\frac{1}{2}\right  )^8\left(\frac{1}{2}\right)^0

    (c) Can you do it?

    Roy
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  3. #3
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    YES! thank you so much

    for C wouldn't I take 1 and subtract the answer of b ?
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  4. #4
    Junior Member roy_zhang's Avatar
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    Quote Originally Posted by Pookie View Post
    YES! thank you so much

    for C wouldn't I take 1 and subtract the answer of b ?
    You got it!
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