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red ,white & green ball

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by prachich1987 » Sat Jan 29, 2011 1:39 am
In a blue jar there are red, white and green balls. The probability of drawing a red ball is 1/5. The probability of drawing a red ball, returning it, and then drawing a white ball is 1/10. What is the probability of drawing a white ball?

a) 1/5.
b) ½.
c) 1/3.
d) 3/10.
e) ¼.

OA : B
source : some online material

I don't understand why P(drawing a red ball.Then keeping it back & then drawing a white ball) should be different from P(drawing a white ball) ?
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by Anurag@Gurome » Sat Jan 29, 2011 1:44 am
prachich1987 wrote:I don't understand why P(drawing a red ball.Then keeping it back & then drawing a white ball) should be different from P(drawing a white ball) ?
That's because the event "drawing a red ball, returning it, and then drawing a white ball" is actually two event. The events are "drawing a red ball and return it" followed by "drawing a white ball".
prachich1987 wrote:In a blue jar there are red, white and green balls. The probability of drawing a red ball is 1/5. The probability of drawing a red ball, returning it, and then drawing a white ball is 1/10. What is the probability of drawing a white ball?

a) 1/5
b) ½
c) 1/3
d) 3/10
e) ¼
P(Drawing a red ball, returning it, and then drawing a white ball) = P(Drawing a red ball, return it)*P(Drawing a white ball)

Hence, 1/10 = (1/5)*P(Drawing a white ball)
=> P(Drawing a white ball) = 1/2

The correct answer is B.
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by yellowho » Mon Jan 31, 2011 7:25 am
This looks like a BTG problem. I think there is a similar problem also on BTG where the OA answer is wrong. I was going to post for discussion but forgot to save. Can you post that question.

[quote="prachich1987"]In a blue jar there are red, white and green balls. The probability of drawing a red ball is 1/5. The probability of drawing a red ball, returning it, and then drawing a white ball is 1/10. What is the probability of drawing a white ball?

a) 1/5.
b) ½.
c) 1/3.
d) 3/10.
e) ¼.

OA : [spoiler]B[/spoiler]
source : some online material

I don't understand why P(drawing a red ball.Then keeping it back & then drawing a white ball) should be different from P(drawing a white ball) ?[/quote]
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