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First of all, I'm sorry if title was not clear, I thought about it for 5 minutes and I couldn't make it better. (English is not my first language)

Ok, now the task is the following. In one class, there are 20 girls and 12 boys. On a revision, the teacher choose with the same probability for each student, one by one, three students. What is the probability the chosen student to be: a) male, b)female if the teacher has chosen 2 girls firstly?

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for a, is it all males? You said we're choosing 3 students but in a) you say the probability of the chosen student is male. Do you mean all males? or the probability of the first student picked is male? –  Eleven-Eleven Apr 17 '13 at 20:29
    
The first 2 chosen are girls. –  user2041143 Apr 17 '13 at 20:30
    
your sample space is 32; 20 girls + 12 boys. for b, if you've already chosen two girls, then your remaining sample space is 30: 18 girls + 12 boys. Does this help? –  Eleven-Eleven Apr 17 '13 at 20:32
    
So, 30:30=1? Sorry, I don't get you, but I got the task done already. :) –  user2041143 Apr 17 '13 at 20:33

2 Answers 2

up vote 2 down vote accepted

Under the given circumstances, there are 18 girsl and 12 boys left as the third choice is made. Thus the probabilities are $\frac{12}{12+18}=\frac25$ and $\frac35$, respectively.

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Wow. That simple? Thanks a lot. –  user2041143 Apr 17 '13 at 20:32

Probability of choosing two girls first = $\frac{20}{32}\frac{19}{31}$. The remaining sample space for the third choice is 30. you should be able to calculate the rest.

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Thanks for your help, but the two girls are chosen firstly by the task itself, I don't have to calculate that, although I've done the rest. :) –  user2041143 Apr 17 '13 at 20:35
    
It was more of a formality doing the probability of the two girls first. –  Eleven-Eleven Apr 17 '13 at 20:39

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