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Math Help - Convinced website is incorrect re Probability and Ratio

  1. #1
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    Convinced website is incorrect re Probability and Ratio

    This website has a quiz about probability and odds: Probability and Odds

    It asks ten questions about the following scenario:

    There are 20 marbles in a bag, 4 blue, 4 green, 4 red, 4 white and 4 black. Remember that all fraction answers should be given in the simplest form.
    The first question it asks is:

    What is the probability of pulling out a red marble?
    Which is pretty straightforward of course it's 4/20 or 1/5.

    Question 2 is about odds rather than probability and it asks:

    What are the odds of pulling out a red marble?
    For this, I worked out that the odds/ratio was 16:4 or (simplified) 4:1. However, the website lists the correct answer as 1/3 (or 1:3).

    Surely website is wrong here?
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  2. #2
    MHF Contributor
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    Re: Convinced website is incorrect re Probability and Ratio

    it's pretty clear

    Pr[picking red marble] = 1/5
    Pr[not picking red marble] = 4/5

    odds of picking the red marble are the ratio of these or, using the common notation, 1:4 or 4:1 against.
    Thanks from emakarov
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