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September 20, 2014

September 20, 2014

Posted by **Anonymous** on Monday, February 18, 2013 at 8:22am.

Two balls are drawn. If probability of both being green is 1/7 find m.

- maths-urgently needed(probability) -
**Reiny**, Monday, February 18, 2013 at 8:44amI will assume the first ball drawn is NOT returned.

prob(2green) = (n/(9+n) * ((n-1)/(8+n)) = 1/7

7n^2 - 7n = n^2 + 17n + 72

6n^2 - 24n - 72 = 0

n^2 - 4n - 12 = 0

(n-6)(n+2) = 0

n = 6 or n = a negative, which is no good

there were 6 green balls

check:

prob(2greens) = (6/15)(5/14) = 1/7

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