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Manager  B
Joined: 26 Feb 2018
Posts: 76
Location: United Arab Emirates
GMAT 1: 710 Q47 V41 GMAT 2: 770 Q49 V47 Show Tags

Method for this question

When ever you see "at least" this is a hint for using the formula P (at least) = 1 - (P less than) (which is easier than considering all the individual scenarios for 2, 3 or 4 matching socks and summing them. That would be very time consuming as we could have WWWW, WWWB, WWWG, WWBB, WWGG WWBG BBWG BBGG BBBW BBBG GGBW etc)

So what is the probability of NOT picking at least 2 socks of the same colour? IE less than 2 matching socks? Ie. NO matching socks. For that we need all 4 socks of a different colour. However, we only have 3 colours, so the probability of this is zero. 1-0=1
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Joined: 18 Jul 2018
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Bunuel wrote:
Official Solution:

There are 5 pairs of white, 3 pairs of black and 2 pairs of grey socks in a drawer. If four individual socks are picked at random what is the probability of getting at least two socks of the same color?

A. $$\frac{1}{5}$$
B. $$\frac{2}{5}$$
C. $$1$$
D. $$\frac{3}{5}$$
E. $$\frac{4}{5}$$

No formula is need to answer this question. The trick here is that we have only 3 different color socks but we pick 4 socks, which ensures that in ANY case we'll have at least one pair of the same color (if 3 socks we pick are of the different color, then the 4th sock must match with either of previously picked one). P=1.

Hi Xylan

I understand logic works perfectly here!

What about complement rule??

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Status: The darker the night, the nearer the dawn!
Joined: 16 Jun 2018
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2
JIAA wrote:
Bunuel wrote:
Official Solution:

There are 5 pairs of white, 3 pairs of black and 2 pairs of grey socks in a drawer. If four individual socks are picked at random what is the probability of getting at least two socks of the same color?

A. $$\frac{1}{5}$$
B. $$\frac{2}{5}$$
C. $$1$$
D. $$\frac{3}{5}$$
E. $$\frac{4}{5}$$

No formula is need to answer this question. The trick here is that we have only 3 different color socks but we pick 4 socks, which ensures that in ANY case, we'll have at least one pair of the same color (if 3 socks we pick are of the different color, then the 4th sock must match with either of previously picked one). P=1.

Hi Xylan

I understand logic works perfectly here!

What about complement rule??

Given Data:
10 White | 6 Black | 4 Grey

JIAA By the complement rule:
Total = Case-1 (when probability is at least 2) + Case-2 (When probability is less than 2)

To purposefully find the probability for which we get less than 2 socks of the same color, Let's fill the 3 of the 4 empty spaces:
W - B - G - __ 4th-space __

As we can see, the 4th-space can ONLY be filled by any of the already existing colors of the socks , i.e., by White, Black OR Grey, thus NO scenario is possible that can fit the conditon.
Hence, the probability for which we get less than 2 socks of the same color = $$0$$ - The Complementing Case

Therefore, ALL the possible cases have at least 2 socks of the same colors, i.e., Probability = 1
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Manager  B
Joined: 18 Jul 2018
Posts: 52
Location: United Arab Emirates

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Xylan wrote:
JIAA wrote:
Bunuel wrote:
Official Solution:

There are 5 pairs of white, 3 pairs of black and 2 pairs of grey socks in a drawer. If four individual socks are picked at random what is the probability of getting at least two socks of the same color?

A. $$\frac{1}{5}$$
B. $$\frac{2}{5}$$
C. $$1$$
D. $$\frac{3}{5}$$
E. $$\frac{4}{5}$$

No formula is need to answer this question. The trick here is that we have only 3 different color socks but we pick 4 socks, which ensures that in ANY case, we'll have at least one pair of the same color (if 3 socks we pick are of the different color, then the 4th sock must match with either of previously picked one). P=1.

Hi Xylan

I understand logic works perfectly here!

What about complement rule??

Given Data:
10 White | 6 Black | 4 Grey

JIAA By the complement rule:
Total = Case-1 (when probability is at least 2) + Case-2 (When probability is less than 2)

To purposefully find the probability for which we get less than 2 socks of the same color, Let's fill the 3 of the 4 empty spaces:
W - B - G - __ 4th-space __

As we can see, the 4th-space can ONLY be filled by any of the already existing colors of the socks , i.e., by White, Black OR Grey, thus NO scenario is possible that can fit the conditon.
Hence, the probability for which we get less than 2 socks of the same color = $$0$$ - The Complementing Case

Therefore, ALL the possible cases have at least 2 socks of the same colors, i.e., Probability = 1

THANK YOU Xylan

MUCH APPRECIATED !!!
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Concentration: Technology, Strategy
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GMAT 1: 560 Q41 V26 GMAT 2: 550 Q43 V23 GMAT 3: 650 Q47 V33 GMAT 4: 650 Q44 V36 WE: Management Consulting (Consulting)

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I got this wrong, but I didn't think it was this simple when reviewing the explanation.

I think a lot of people are over complicating this.

There are 10 White, 6 Blue and 4 Grey.

What is the probability that I pick a matching pair of socks if I am to choose 4 individual socks?
4 Choices: _ _ _ _

Randomize this: W B G ?
? = we must choose either White, Blue or Grey. So we have a 100% chance of picking a matching pair we choose
W B G (W/B/G) = Matching pair.
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Intern  B
Joined: 12 Nov 2016
Posts: 7

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chetan2u wrote:
MEOWSER wrote:
Bunuel,

Thanks for the question. If it were phrased differently and we only picked 3 individual socks instead of 4 individual socks, how would we approach the question?

the recommended way would be..
in this case you will have to find the probability of all three being different...
since there are three different socks , the ways to pick up three amongst th three would be 3!=6...
now the probability in each way=$$\frac{10}{20}*\frac{6}{19}*\frac{4}{18}$$=$$\frac{2}{57}$$...
so for 6 ways=6*$$\frac{2}{57}$$= $$\frac{12}{57}$$..
so the prob of not having all different or in other words having atleast two of same colour= 1-$$\frac{12}{57}$$=$$\frac{45}{57}$$

Hello
Please explain how does the selection order (and thereby an extra factor of 3!) matter in the above solution provided if we were to make a selection of 3 individual socks.

Thanks Re: M02-06   [#permalink] 11 Jun 2019, 05:58

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