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The probability of pulling a black ball out of a glass jar

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The probability of pulling a black ball out of a glass jar [#permalink] New post 17 Dec 2010, 04:39
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The probability of pulling a black ball out of a glass jar is 1/X. The probability of pulling a black ball out of a glass jar and breaking the jar is 1/Y. What is the probability of breaking the jar?

a) 1/(XY).
b) X/Y.
c) Y/X.
d) 1/(X+Y).
e) 1/(X-Y).
[Reveal] Spoiler: OA
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Re: The probability of pulling a black ball out of a glass jar [#permalink] New post 17 Dec 2010, 04:48
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anilnandyala wrote:
The probability of pulling a black ball out of a glass jar is 1/X. The probability of pulling a black ball out of a glass jar and breaking the jar is 1/Y. What is the probability of breaking the jar?

a) 1/(XY).
b) X/Y.
c) Y/X.
d) 1/(X+Y).
e) 1/(X-Y).



Probability of A and B.
When two events are independent, the probability of both occurring is the product of the probabilities of the individual events: P(A \ and \ B) = P(A)*P(B)

P(break & black) = P(break) * P(black) --> 1/y=P(break)*1/x --> P(break)=x/y.

Answer: B.
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Re: The probability of pulling a black ball out of a glass jar [#permalink] New post 17 Dec 2010, 23:38
pretty new to this forum and have a question. do you not use plug in or substitution and try to use just the math specifically?

If substitute here, much easier, i think. 1/x substitute x=5 1/y = substitute y = 6 (it has to be less likely to break and black ball, otherwise, there wouldnt be a black ball in the jar) then you have...

1/5 * P(both) = 1/6

5(1/5 * P(both)) = (1/6)5

P(both) = 5/6
Re: The probability of pulling a black ball out of a glass jar   [#permalink] 17 Dec 2010, 23:38
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