A bag contains six white and four red balls. Four balls are drawn one-by-one without replacement and it is found that there are at least two red balls drawn.
The probability that the next draw of a ball from this bag is a red ball can be expressed as b a , where a and b are coprime positive integers. Find a + b .
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Although I am not able to find any mistake in your solution, I still feel the answer should have been 34/210 or 17/105.
I am getting a little confused about why conditional probability formula was required in the last step when we are already going step by step while calculating AΩB :/
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The probability 2 1 0 3 4 , or 1 0 5 ! 7 is the probability P [ A ∩ B ] that at least two out of first 4 , plus the fifth, balls drawn are red. This is not the same as the conditional probability P [ B ∣ A ] of the fifth ball being red, given that we know that at least two of the first four balls are red.
The question tells us that we know that A has happened, and we then interested in the outcome of the fifth ball. This is a conditional probability situation.
Think of a similar problem. Deal one card from the top of the deck. You see that it is an Ace. What is the probability that the next card off the top is another Ace? This is 5 1 3 = 1 7 1 , and not 5 2 4 × 5 1 3 = 2 2 1 1 , which is the probability that the top two cards are Aces.
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I also get the result 17/105 considering that the first 4 balls contain 2 or 3 red balls and the fifth one must have red colour! What should there be wrong?
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@Andreas Wendler – As I told Rohit, you are working out the probability that both at least two of the first four are red, and also that the fifth is red. This is not the same as the probability that the fifth is red, given that at least two our of the first four are red. Read up on conditional probability.
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@Mark Hennings – Yes, I thought the total probability is meant. But it's clear that it must differ from the condititional probability! The task is a nice trap!
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@Andreas Wendler – Nevertheless the task did not point out that a conditional probability has to be calculated!?
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@Andreas Wendler – Yes it does. You are told that at least two of the first four are red. All other probabilities then have to be calculated conditional on that knowledge. In just the same way, the problem "You deal one card off the top of a deck and get an ace; what is the probability that the next card is also an ace?" requires you to find a conditional probability, without phrases like "conditional" or "given that" being explicitly stated.
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@Mark Hennings – But I'm afraid it should be very difficult to prove the result empirically although the mathematical correctness is surely obvious!?
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@Andreas Wendler – Not really. You could conduct a computer simulation of drawing five balls out of the bag, and discarding all outcomes for which there are fewer than two red balls in the first four. The (experimental) probability to look for is then the proportion of the remaining tests that have red fifth ball.
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@Mark Hennings – Oh yes, via random generator with a sufficient high count of trials this will work!
Thanks a lot! Just a little bit change of words & the question gets twisted when playing with probability! The "similar problem" was a good example :)
Thanks a lot sir! :) I also did that mistake :P .....
Or otherwise...could u frame a question for me whose answer will come out to be 17/105 ??
@Calvin Lin : sir any comments?
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What Mark wrote looks good to me. His "similar problem" example is also one that I would given.
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Let A j be the event that j of the first 4 picks are red (for 2 ≤ j ≤ 4 ). Let A be the event that at least 2 of the first 4 picks are red, and let B be the event that the fifth pick is red. Certainly,then, A = A 2 ∪ A 3 ∪ A 4 . Thus P [ A ] P [ A ∩ B ] = P [ A 2 ] + P [ A 3 ] + P [ A 4 ] = ( 4 1 0 ) ( 2 4 ) ( 2 6 ) + ( 3 4 ) ( 1 6 ) + 1 = 2 1 0 1 1 5 = P [ A 2 ∩ B ] + P [ A 3 ∩ B ] + P [ A 4 ∩ B ] = ( 4 1 0 ) ( 2 4 ) ( 2 6 ) × 3 1 + ( 4 1 0 ) ( 3 4 ) ( 1 6 ) × 6 1 + ( 4 1 0 ) 1 × 0 = 2 1 0 3 4 and hence P [ B ∣ A ] = P [ A ] P [ A ∩ B ] = 1 1 5 3 4 making the answer 1 4 9 .