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

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Director
Joined: 22 Aug 2007
Posts: 572
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The probability of pulling a black ball out of a glass jar [#permalink]  22 Sep 2007, 08:31
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).
VP
Joined: 10 Jun 2007
Posts: 1465
Followers: 6

Kudos [?]: 133 [0], given: 0

Re: combination_break it [#permalink]  22 Sep 2007, 08:55
IrinaOK 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).

I got b
P(black)=1/x
P(break and black)=1/y=P(black)xP(break)
P(break)= x/y
Director
Joined: 11 Jun 2007
Posts: 932
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Re: combination_break it [#permalink]  22 Sep 2007, 08:57
IrinaOK 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).

A = probability of pulling a black ball out of a glass jar 1/X
B = probability of breaking the jar
C = pulling a black ball out of a glass jar and breaking the jar 1/Y

A * B = C

solve for B: B = C / A
B = (1/Y) / (1/X)
B = X /Y (answer B)
Current Student
Joined: 28 Dec 2004
Posts: 3391
Location: New York City
Schools: Wharton'11 HBS'12
Followers: 13

Kudos [?]: 180 [0], given: 2

i got B too...

prob of pulling ball 1/x

prob of breaking jar 1/n

prob of pulling ball AND breaking jar 1/y

1/x * 1/n =1/y

1/n=x/y
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