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    Justin1932's Avatar
    Justin1932 Posts: 2, Reputation: 1
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    #1

    Feb 24, 2010, 06:08 PM
    probability equations
    I am having some trouble with statistics and there isn't anything in my book to help me.

    A Binary String is a sequence of digits either 0 or 1.

    a.) How many binary strings of length 9 are there? (The first digit may be zero).

    b.) How many binary strings of length 9 contain the digit 1 exactly 3 times?

    c.) How many binary strings of length 9 alternate 0’s and 1’s?

    d.) How many binary strings of length 9 have an even number of 1’s?
    Unknown008's Avatar
    Unknown008 Posts: 8,076, Reputation: 723
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    #2

    Feb 25, 2010, 02:49 AM

    a) Imagine you are told to draw a probability tree for that. I hope you are quite familiar with probability trees.

    On the first digit, you have either 0 or 1. (2 possibilities or 2^1)
    Upon the second digit, you have the sequence becoming 00, 01, 10 or 11. (4 possibilities or 2^2)
    Upon the third digit, you have the sequence becoming 000, 001, 010, 011, 100, 101, 110 or 111. (8 possibilities or 2^3)
    Do you find a sequence here? Each time, the number of possibilities is doubled. Hence, you can say that for a sequence of length 10 has 2^5 = 32 possibilities.

    Post your answer! :)

    b) Here, you start with the Binomial Distribution.

    Let X be the probability that a one is in the sequence.
    X ~ B(0.5, 9)

    [0.5 is the probability of having a 1 when the length of the sequence is 1 and 9 is the length of the sequence]

    So,

    You can do the math.

    c) I'm not sure of this one, but alternate 0's and 1's means either 010101010 or 101010101. So I see only 2. The probability of having that is 2/[Total number of binary strings]

    d) This one, you can find it using:

    P(X = 2) + P(X = 4) + P(X = 6) + P(X = 8)

    Post your answers! :)

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