17. A group consists of 7 men and some women. The probability of
selecting 2 women from them is 1s. Find the number of women in
the group
[Ans. : 3]
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3
Answer:
There are 3 women in the group.
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Step-by-step explanation:
Let w be the number of women.
The number of people is then w+7.
P(2 people chosen are women) = 1/15
=> ( # ways to choose 2 women ) / ( # ways to choose 2 people ) = 1 / 15
=> 15 × ( # ways to choose 2 women ) = # ways to choose 2 people
=> 15 w ( w - 1 ) / 2 = ( w + 7 ) ( w + 6 ) / 2
=> 15w² - 15w = w² + 13w + 42
=> 14w² - 28w - 42 = 0
=> w² - 2w - 3 = 0
=> ( w - 3 ) ( w + 1 ) = 0
=> w = 3 ( since the number of women is non-negative).
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