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Number of rolls needed

  1. Nov 25, 2007 #1
    I'm confused about this question.

    The problem: The sum of the rolls of a fair die exceed 300. Find the probability that at least 80 rolls were necessary.

    The solution we were given is that this event is equivalent to the sum of 79 rolls being less than or equal to 300. But I don't get it. The sum of 79 rolls could be 79, assuming you rolled a one every time. Then the 80th roll would not exceed 300. There are plenty of cases of the sum of 79 rolls being less than 300 where the 80th roll would not exceed 300. Can anyone help explain this to me?

  2. jcsd
  3. Nov 25, 2007 #2
    AT LEAST 80 rolls. That means 80 or MORE. Your example has more than 80 rolls. So it is at least 80 rolls.
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