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Probability - 7 students

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ramyaravindran: Senior | Next Rank: 100 Posts; Posts: 49; Joined: Mon Apr 28, 2008 5:33 am; Thanked: 2 times

by ramyaravindran » Sun Jan 17, 2010 12:43 pm

In a class of 20 students, one half of the students are boys, if a teacher has to select 7 students and the first 6 are girls.What is the probability that the 7th student is a girl

The given answer is [spoiler]2/7[/spoiler]

Can somebody explain how to solve this question?Thanks.[/spoiler]

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Source: Beat The GMAT — Problem Solving | Sun Jan 17, 2010 12:43 pm

bln123: Senior | Next Rank: 100 Posts; Posts: 59; Joined: Mon Jul 20, 2009 6:39 amFollowed by: 1 member; Thanked: 4 times

by bln123 » Sun Jan 17, 2010 2:11 pm

[spoiler]20 Students, 10 male, 10 female

6 are already picked, all girls -> remaining students: 4 female, 10 male

probability of 7th pick being female: 4 (number of females remaining) / 14 (number of all students remaining)

4/14 -> 2/7 (-> around 28%)[/spoiler]

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