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black and white socks

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by sanju09 » Sat Jul 24, 2010 4:20 am
Two socks are to be picked at random from a drawer containing only black and white socks. What is the probability that both are white?

1. The probability of the first sock being black is 1/3.
2. There are 24 white socks in the drawer.

[spoiler]Source: majortests.com[/spoiler]
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Source: — Data Sufficiency |
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by clock60 » Sat Jul 24, 2010 5:19 am
got C here
(1) insuffiicent as we don`t know the number of white
(2) insufficient no numberof black
both sufficient
(black)/(black+24)=1/3
black=12, from here we can calculate the prob of picking two white
(24C2)/36C2)
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by indiantiger » Sat Jul 24, 2010 4:59 pm
1) insuff does not talk about probability of white socks, only info given is nb/(nw+nb) = 1/3

2) total white socks = 24, no info about black socks
insuff


combine 1 and 2


nw = 24

nb/24+nb = 1/3

this will give us nb.

Once we have total number of Black socks and total number of white socks we can calculate the desired probability. As this is a DS question, getting the right value is not necessary.
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