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ChanSu

Not sure where I’m going wrong

(7C2 x 5C1) / 12 C3
=21/440

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The process seems correct. You've probably ended up making an arithmetic error.

\(^7C_2 =\frac{ 7 *6 }{ 2} = 21\)

\(^5C_1 = 5 \)

\(^{12}C_3 = \frac{12*11*10}{3*2} = 2 * 11*10 \)

\(\frac{^{7}C_2 \quad * \quad ^{5}C_1 }{ ^{12}C_3} = \frac{21 * 5 }{ 2 * 11 * 10} = \frac{21 }{ 44}\)
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gmatophobia
ChanSu

Not sure where I’m going wrong

(7C2 x 5C1) / 12 C3
=21/440

Posted from my mobile device

The process seems correct. You've probably ended up making an arithmetic error.

\(^7C_2 =\frac{ 7 *6 }{ 2} = 21\)

\(^5C_1 = 5 \)

\(^{12}C_3 = \frac{12*11*10}{3*2} = 2 * 11*10 \)

\(\frac{^{7}C_2 \quad * \quad ^{5}C_1 }{ ^{12}C_3} = \frac{21 * 5 }{ 2 * 11 * 10} = \frac{21 }{ 44}\)
Thank you . Yes I made an Arthematic error
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A question : why are we considering the order here? Nothing in the question stem suggests the ask for order (or does it)?gmatophobia
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DebduhitaB
A question : why are we considering the order here? Nothing in the question stem suggests the ask for order (or does it)?gmatophobia
Hi Quant experts - pls. share reply on the above query
Also, can you please clarify how do we be sure in the questions that the orders matters or not?
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DebduhitaB
A bag has 5 red balls and 7 white balls. Three balls are drawn randomly from the bag (without replacement). What is the probability that two of the balls drawn are white and one is red?

A. \(\frac{7}{44}\)

B. \(\frac{7}{22}\)

C. \(\frac{21}{44}\)

D. \(\frac{21}{22}\)

E. \(\frac{35}{110}\)

A question : why are we considering the order here? Nothing in the question stem suggests the ask for order (or does it)?gmatophobia

The question asks to find the probability of drawing two white balls and one red ball, regardless of the order. That's the crucial point. If a specific order, like WWR (white-white-red), was required, the calculation would be 7/12 * 6/11 * 5/10. But since order is not specified, we consider all possible sequences: WWR, WRW, RWW. Each sequence has the same probability, so we calculate the probability for one sequence and then multiply by the number of sequences to get the total probability.

Hope it's clear.
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total combinations = 12c3 = 440
RWW = 5c1*7c1*6c1 = 35*6
= 210/440= 21/44
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