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Math Help - light bulbs

  1. #1
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    light bulbs

    Suppose you have 100 light bulbs and one of them is defective. If you pick out two light bulbs at random (either borh at the same time, or first one, then another, without replacing the first light bulb), what is the probability that one of your chosen light bulbs is defective?

    i got an answer, but it seemed too easy

    1/100 + (1/100)(1/99)

    is this right?
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  2. #2
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    isnt it just 1/50?

    1 - P(Not being defective) = 1 - (99/100).(98/99) = 1/50
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  3. #3
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    Consider how we could get one good and one bad: GB\text{ or }BG.
    What are those probabilities: \frac{99}{100}\frac{1}{99} +\frac{1}{100} \frac{99}{100}=?
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  4. #4
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    Quote Originally Posted by ihavvaquestion View Post
    Suppose you have 100 light bulbs and one of them is defective. If you pick out two light bulbs at random (either borh at the same time, or first one, then another, without replacing the first light bulb), what is the probability that one of your chosen light bulbs is defective?

    i got an answer, but it seemed too easy

    1/100 + (1/100)(1/99)

    is this right?
    You have summed the probabilities of getting the bad one first
    and getting the bad one first followed by a "specific" good one.

    You need the bad one first or 2nd, which is BG+GB

    \frac{1}{100}\ \frac{99}{99}+\frac{99}{100}\ \frac{1}{99}

    which follows from ..... pick one bulb then another.
    After the first bulb is picked there are 99 to choose from.
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