Basic probability question

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WlND

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Dec 12, 2011
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Hi,

Its been a couple years since I have done stats so I'm kind of rusty.

There is a bag of 6 balls. 3 are purple and 3 are green. What is the probability of getting at least 2 purple balls in 3 draws?

probability of getting 2 purple balls

3/6*2/5*3/4=0.15

probability of getting 3 purple balls

3/6+ 2/5+1/4=0.05

0.15+0.05=0.20

therefore, the probability of drawing at least 2 purple balls is 20%

*the question doesn't say, but I'm assuming that the balls are not being replaced

its that correct?
 
WlND said:
Hi,

Its been a couple years since I have done stats so I'm kind of rusty.

There is a bag of 6 balls. 3 are purple and 3 are green. What is the probability of getting at least 2 purple balls in 3 draws?

probability of getting 2 purple balls

3/6*2/5*3/4=0.15....Since we don't care which of the three is green,
..............................this probability should be multiplied by 3

probability of getting 3 purple balls

3/6+ 2/5+1/4=0.05

0.15+0.05=0.20

therefore, the probability of drawing at least 2 purple balls is 20%

*the question doesn't say, but I'm assuming that the balls are not being replaced

its that correct?
Click to expand...
3*0.15 + 0.05 = 0.50

If you look at the symmetry of the setup, It has to be true that one or the other color has at least 2. It is equally likely that the majority will be green or purple. Thus it is 50% probability to have 2 or more purple.
 
By the way, you don't say if this is "with replacement" or "without replacement". Your calculations indicate "without replacement".
 
^ the question didn't specify so I just assumed that it was without replacement
 
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