probablity ques

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nitug: Junior | Next Rank: 30 Posts; Posts: 12; Joined: Wed Jul 23, 2008 2:10 am

by nitug » Sat Aug 30, 2008 12:18 pm

hi all,

i almost always get probabilty ques wrong..plz do help me guys, till then i m looking and exploring the forums if someone has already pointed out the strategies to improve it..

Also solve this one ques for me,

The bowl contains chips of red and green color and no chips of any other color. If two chips are drawn from the basket at random without replacement, what is the probability that both chips will be green?
1. 20% of all chips in the basket are green
2. The ratio of the number of red chips to the number of green chips is 4:1

• Statement (1) ALONE is sufficient, but Statement (2) ALONE is not sufficient
• Statement (2) ALONE is sufficient, but Statement (1) ALONE is not sufficient
• BOTH statements TOGETHER are sufficient, but NEITHER statement ALONE is sufficient
• EACH statement ALONE is sufficient
• Statements (1) and (2) TOGETHER are NOT sufficient

i really appreciate this forum and its members!!

thanks,
nitug

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Source: Beat The GMAT — Data Sufficiency | Sat Aug 30, 2008 12:18 pm

sudhir3127: Legendary Member; Posts: 829; Joined: Mon Jul 07, 2008 10:09 pmFollowed by: 3 members; Location: INDIA; Thanked: 84 times

Re: probablity ques

by sudhir3127 » Sat Aug 30, 2008 1:01 pm

nitug wrote:hi all,

i almost always get probabilty ques wrong..plz do help me guys, till then i m looking and exploring the forums if someone has already pointed out the strategies to improve it..

Also solve this one ques for me,

The bowl contains chips of red and green color and no chips of any other color. If two chips are drawn from the basket at random without replacement, what is the probability that both chips will be green?
1. 20% of all chips in the basket are green
2. The ratio of the number of red chips to the number of green chips is 4:1

• Statement (1) ALONE is sufficient, but Statement (2) ALONE is not sufficient
• Statement (2) ALONE is sufficient, but Statement (1) ALONE is not sufficient
• BOTH statements TOGETHER are sufficient, but NEITHER statement ALONE is sufficient
• EACH statement ALONE is sufficient
• Statements (1) and (2) TOGETHER are NOT sufficient

i really appreciate this forum and its members!!

thanks,
nitug

IMO E

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LSB: Master | Next Rank: 500 Posts; Posts: 229; Joined: Thu Nov 08, 2007 3:47 pm; Thanked: 14 times; GMAT Score:750

by LSB » Sat Aug 30, 2008 1:34 pm

I think E as well. Both statements say the same thing. that 1 out of balls is green. However, this is a dependent prob question and we need to know how many balls are left once one is removed (or what the new ratios will be once one is removed)

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sethids: Master | Next Rank: 500 Posts; Posts: 135; Joined: Mon Jun 23, 2008 3:56 am; Thanked: 7 times

by sethids » Sat Aug 30, 2008 1:36 pm

Both the statements here give the same information.
20% chips are green, therefore 80% are red. Hence the ratio of red : green is 4 : 1.

Had it been a question of selection with replacement then the ratio would have been sufficient to answer. The probability of getting a green chip in one selection would have been 1/5. Because there are 2 selections, the probability of those 2 events happening simultaneously would have been 1/5 * 1/5 = 1/25.

But, here once we have made a selection (which still holds a probability of 1/5). We cannot determine the probability of the next selection unless we know the number of red and green chips or the modified ratio after one selection has been made.

Hence E.

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nitug: Junior | Next Rank: 30 Posts; Posts: 12; Joined: Wed Jul 23, 2008 2:10 am

by nitug » Mon Sep 01, 2008 9:15 am

ans is E and again i made the mistake of not understanding the ques completely and missed the words "without replacement"
thanks for pointing that out sethids.

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