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# A jar contains only x black balls and y white balls

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Director
Joined: 25 Oct 2008
Posts: 607
Location: Kolkata,India
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Kudos [?]: 428 [0], given: 100

A jar contains only x black balls and y white balls [#permalink]  06 Jun 2009, 03:54
00:00

Difficulty:

5% (low)

Question Stats:

86% (01:43) correct 14% (01:00) wrong based on 80 sessions
A jar contains only x black balls and y white balls. One ball is drawn randomly from the jar and is not replaced. A second ball is then drawn randomly from the jar. What is the probability that the first ball drawn is black and the second ball drawn is white?

A. $$\frac{x}{x+y}*\frac{y}{x+y}$$

B. $$\frac{x}{x+y}*\frac{x-1}{x+y-1}$$

C. $$\frac{xy}{x+y}$$

D. $$\frac{x-1}{x+y}*\frac{y-1}{x+y}$$

E. $$\frac{x}{x+y}*\frac{y}{x+y-1}$$
[Reveal] Spoiler: OA

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Manager
Joined: 12 Apr 2006
Posts: 218
Location: India
Followers: 1

Kudos [?]: 24 [2] , given: 17

Re: A jar contains only x black balls and y white balls [#permalink]  06 Jun 2009, 08:09
2
KUDOS
Black balls = x
White balls = y

Total no of balls = x + y

Probability of first ball drawn is black = x/(x+y)
As the ball is not replaced now the total remaining balls = (x+y-1)

Probability of second ball drawn is white = y/(x+y-1)

Total probability = $$[\frac{x}{(x+y)}]*[\frac{y}{(x+y-1)}]$$
Director
Status: No dream is too large, no dreamer is too small
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Kudos [?]: 487 [0], given: 39

Re: A jar contains only x black balls and y white balls [#permalink]  10 Jan 2012, 06:18
Retired Moderator
Status: 2000 posts! I don't know whether I should feel great or sad about it! LOL
Joined: 04 Oct 2009
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Schools: Harvard, Stanford, Wharton, MIT & HKS (Government)
WE 1: Economic research
WE 2: Banking
WE 3: Government: Foreign Trade and SMEs
Followers: 81

Kudos [?]: 587 [0], given: 109

Re: A jar contains only x black balls and y white balls [#permalink]  21 Feb 2012, 13:02
It seems difficult, but actually is one of the easiest.
Just do it.

$$\frac{x}{(x+y)} * \frac{y}{(x+y-1)}$$

+1 E
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Re: A jar contains only x black balls and y white balls   [#permalink] 21 Feb 2012, 13:02
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