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    jamester0941's Avatar
    jamester0941 Posts: 4, Reputation: 1
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    #1

    Feb 15, 2013, 11:27 AM
    Statistics question
    Ok brainiacs, a little help is greatly appreciated.

    A student has a box containing 25 computer disks, of which 15 are blank
    and the other 10 are not. If student randomly selects disks one by one, what is
    the probability that at least two must be selected in order to find one that is
    blank? (2 pts)

    Please show work!
    ebaines's Avatar
    ebaines Posts: 12,131, Reputation: 1307
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    #2

    Feb 15, 2013, 11:33 AM
    The probability that "at least two" must be selected to get a blank is one minus the probability that only one needs to be selected, which is one minus the probability that the first disk selected is blank.

    That's enough of a hint - can you finish it from here?
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    jamester0941 Posts: 4, Reputation: 1
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    #3

    Feb 15, 2013, 11:49 AM
    Quote Originally Posted by ebaines View Post
    The probability that "at least two" must be selected to get a blank is one minus the probability that only one needs to be selected, which is one minus the probability that the first disk selected is blank.

    That's enough of a hint - can you finish it from here?
    how does that equation look though? That's what's killing me
    ebaines's Avatar
    ebaines Posts: 12,131, Reputation: 1307
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    #4

    Feb 15, 2013, 12:04 PM
    Quote Originally Posted by jamester0941 View Post
    how does that equation look though? thats whats killin me
    Hint, usiing a similar problem - if you have 5 red candies out of 12 in a box the probability of choosing a red candy is 5/12. This is because out of the 12 candies only 5 meet the criteria of being red. Now apply this same thinking to your problem - what's the probability of selecting a blank disk when there are 15 blank disks out of 25?

    BTW, another way to approach this problem is to realize that if at least two disks must be selected that means the first disk was not blank. So this problem is really just asking: what's the probability of selecting a non-blank disk on the first try?

    OK - back to you.
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    jamester0941 Posts: 4, Reputation: 1
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    #5

    Feb 15, 2013, 12:12 PM
    Guy, I just wanted someone to answer the question for me. Not try to guide me to figuring it out myself. I have a professor that gets paid good money to do that. I give you a helpful because you did respond but this stats class is a check the box selection on my degree plan. I'm reallly not the least bit concerned at becoming a pro at this stuff. Thanks
    ebaines's Avatar
    ebaines Posts: 12,131, Reputation: 1307
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    #6

    Feb 15, 2013, 12:47 PM
    Quote Originally Posted by jamester0941 View Post
    Guy, I just wanted someone to answer the question for me. Not try to guide me to figuring it out myself. I have a professor that gets paid good money to do that.
    Sorry - it's against our policy to simply give answers, especially to test questions. So try to solve it on your own. Actually this question is hardly worthy of being called "statistics" - it's more like a simple logic question that anybody in a degree program should be able to think through in about a minute. I am happy to help if you get stuck, but you must at least show an attempt to solve it.
    jamester0941's Avatar
    jamester0941 Posts: 4, Reputation: 1
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    #7

    Feb 15, 2013, 01:02 PM
    Its not a test question. Not sure how you concluded that. Its actually an extra credit question. Not even a required assignment. Oh, if only I could access the internet while taking a test in this class. My life would be perfect then!
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