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Two positive integers have a sum of 9, what is the probability that one of them was 7.

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lxsptter | Student, Undergraduate | Valedictorian

Posted January 20, 2012 at 2:14 PM via web

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Two positive integers have a sum of 9, what is the probability that one of them was 7.

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justaguide | College Teacher | (Level 2) Distinguished Educator

Posted January 20, 2012 at 2:21 PM (Answer #1)

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Two positive integers have a sum of 9. The number of positive number pairs that add up to 9 are (8, 1), (1, 8), (7, 2), (2, 7), (3, 6), (6, 3), (4, 5), (5, 4)

There are 8 pairs that add up to 9. Of these 2 of them have the integer 7. This gives a probability of 2/8 = 0.25

The probability that one of the two positive integers that add up to 9 being 7 is 0.25

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William Delaney | (Level 3) Educator Emeritus

Posted January 21, 2012 at 12:11 PM (Answer #4)

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.25 is correct but there are only four possible combinations: 1 and 8, 3 and 6, 4 and 5. These are not dice but numbers. 5 and 4 are the same numbers as 4 and 5, etc. There is only one probability that one of the numbers would be 7. It doesn't matter which one.

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