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# Probability that the birthdays of 6 different persons will fall in exa

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Probability that the birthdays of 6 different persons will fall in exa [#permalink]

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28 Apr 2011, 05:42
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34% (01:44) correct 66% (19:54) wrong based on 41 sessions

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Probability that the birthdays of 6 different persons will fall in exactly 2 calendar months is

A. 341/12^6
B. 352/12^6
C. 352/12^5
D. 341/12^5
E. 371/12^6
[Reveal] Spoiler: OA

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Re: Probability that the birthdays of 6 different persons will fall in exa [#permalink]

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28 Apr 2011, 07:27
Probability that the birthdays of 6 different persons will fall in exactly 2 calendar months is

A. 341/12^6
B. 352/12^6
C. 352/12^5
D. 341/12^5
E. 371/12^6

Dont even have a clue how to approach this one

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Re: Probability that the birthdays of 6 different persons will fall in exa [#permalink]

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28 Apr 2011, 07:32
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Probability that the birthdays of 6 different persons will fall in exactly 2 calendar months is

A. 341/12^6
B. 352/12^6
C. 352/12^5
D. 341/12^5
E. 371/12^6

Sol:

Assuming that any person having a birthday on any month to be $$\frac{1}{12}$$. In reality; probability of a person having a birthday on Feb(28 days) is less than having a birthday on January(31 days).

$$C^{12}_2*\frac{2^6-2}{12^6}=\frac{341}{12^5}$$

$$C^{12}_2$$ - Number of ways to choose 2 months out of 12
$$2^6-2$$ - 2 favorable choices(months) available for every person excluding 000000,111111(All Birthdays in the same month)
$$12^6$$ - Every person has 12 Total possible months

Ans: "D"
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Re: Probability that the birthdays of 6 different persons will fall in exa [#permalink]

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28 Apr 2011, 09:54

000000,111111(All Birthdays in the same month)
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Re: Probability that the birthdays of 6 different persons will fall in exa [#permalink]

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28 Apr 2011, 10:05
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subhashghosh wrote:

000000,111111(All Birthdays in the same month)

6 people have two favorable choices each; Let's say the choices were 0(JAN),1(FEB) for simplicity just as it would be for head and tail.

Thus, total number of combinations would be: 2^6.

000000
000001
000010
000011
.
.
111111

Total=2^6

As per the condition that all 6 must have EXACTLY two calender months as their birthdays, at least two people should have different birthdays. They all can't have 000000 i.e. JAN,JAN,JAN,JAN,JAN,JAN or 1,1,1,1,1,1 i.e. FEB,FEB,FEB,FEB,FEB,FEB.

So, Favorable possibilities = 2^6-2=62.
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Re: Probability that the birthdays of 6 different persons will fall in exa [#permalink]

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28 Apr 2011, 18:13
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Expert's post
Probability that the birthdays of 6 different persons will fall in exactly 2 calendar months is

A. 341/12^6
B. 352/12^6
C. 352/12^5
D. 341/12^5
E. 371/12^6

Another slightly different way of approaching this (and a little more complicated than fluke's - which I think is a very appropriate solution. Nevertheless, it might be useful for people who have done permutation combination before).
Select 2 months and then no of people for each month. You select two months in 12C2 ways. Say you get J and F.
Then you say that J could have 1 person's bday, 2 people's bday, 3 people's bday etc

J - 1/2/3/4/5 (You select 1 or 2 or 3 or 4 or 5 people for J)
F - 5/4/3/2/1 (Corresponding to no of people selected for J, remaining people go in F)

The distribution can be done in $$6C1 + 6C2 + 6C3 + 6C4 + 6C5 = 2^6 - 2 = 62$$ ways (If you recognize it, great, else just calculate)

So probability $$= 12C2 * 62/ 12^6 = 341/12^5$$
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Get started with Veritas Prep GMAT On Demand for $199 Veritas Prep Reviews Kudos [?]: 17380 [1], given: 232 Intern Status: It's a long road ahead Joined: 05 Feb 2011 Posts: 3 Kudos [?]: 1 [0], given: 1 Location: Chennai, India Re: Probability that the birthdays of 6 different persons will fall in exa [#permalink] ### Show Tags 28 Apr 2011, 23:43 VeritasPrepKarishma wrote: jamifahad wrote: Probability that the birthdays of 6 different persons will fall in exactly 2 calendar months is A. 341/12^6 B. 352/12^6 C. 352/12^5 D. 341/12^5 E. 371/12^6 Another slightly different way of approaching this (and a little more complicated than fluke's - which I think is a very appropriate solution. Nevertheless, it might be useful for people who have done permutation combination before). Select 2 months and then no of people for each month. You select two months in 12C2 ways. Say you get J and F. Then you say that J could have 1 person's bday, 2 people's bday, 3 people's bday etc J - 1/2/3/4/5 (You select 1 or 2 or 3 or 4 or 5 people for J) F - 5/4/3/2/1 (Corresponding to no of people selected for J, remaining people go in F) The distribution can be done in $$6C1 + 6C2 + 6C3 + 6C4 + 6C5 = 2^6 - 2 = 62$$ ways (If you recognize it, great, else just calculate) So probability $$= 12C2 * 62/ 12^6 = 341/12^5$$ My understanding of your solution goes this way.Let me know if I'm wrong. 1. Selecting any two months out of the 12 available can be done in 12C2 ways. 2. The 6 days can be distributed in the 2 available months in the following ways M1 M2 0 6 1 5 2 4 3 3 4 2 5 1 6 0 which essentially is 6C0 + 6C1 + 6C2 + 6C3 + 6C4 + 6C5 + 6C6, which is of the form (x+a)^6 = x^6 + x^5 6C1 a^1 + x^4 6C2 a^2 + x^3 6C3 a^3 + . . + a^6. Substituting 1 for 'x' and 'a' we arrive at (1+1)^6 = 1 + 6C1 + 6C2 + 6C3 + 6C4 + 6C5 + 1 2^6 = 2 + 6C1 + 6C2 + 6C3 + 6C4 + 6C5 i.e, 2^6-2 = 6C1 + 6C2 + 6C3 + 6C4 + 6C5 = 62 Here, we subtract 2 because we are not to take into account either cases where all 6 birthdays fall in M1 or M2 Thus the probability becomes 12C2 * 62/ 12^6 = 341/12^5 _________________ Deepakh Kudos [?]: 1 [0], given: 1 Director Status: Impossible is not a fact. It's an opinion. It's a dare. Impossible is nothing. Affiliations: University of Chicago Booth School of Business Joined: 03 Feb 2011 Posts: 871 Kudos [?]: 396 [0], given: 123 Re: Probability that the birthdays of 6 different persons will fall in exa [#permalink] ### Show Tags 28 Apr 2011, 23:59 Exactly as the binomial dist you have written! That is basically weeding out all the cases which have "all" 6 birthdays in one month i.e. 6C6 and 6C0 Kudos [?]: 396 [0], given: 123 Veritas Prep GMAT Instructor Joined: 16 Oct 2010 Posts: 7676 Kudos [?]: 17380 [0], given: 232 Location: Pune, India Re: Probability that the birthdays of 6 different persons will fall in exa [#permalink] ### Show Tags 29 Apr 2011, 04:21 deepakh88 wrote: My understanding of your solution goes this way.Let me know if I'm wrong. 1. Selecting any two months out of the 12 available can be done in 12C2 ways. 2. The 6 days can be distributed in the 2 available months in the following ways M1 M2 0 6 1 5 2 4 3 3 4 2 5 1 6 0 which essentially is 6C0 + 6C1 + 6C2 + 6C3 + 6C4 + 6C5 + 6C6, which is of the form (x+a)^6 = x^6 + x^5 6C1 a^1 + x^4 6C2 a^2 + x^3 6C3 a^3 + . . + a^6. Substituting 1 for 'x' and 'a' we arrive at (1+1)^6 = 1 + 6C1 + 6C2 + 6C3 + 6C4 + 6C5 + 1 2^6 = 2 + 6C1 + 6C2 + 6C3 + 6C4 + 6C5 i.e, 2^6-2 = 6C1 + 6C2 + 6C3 + 6C4 + 6C5 = 62 Here, we subtract 2 because we are not to take into account either cases where all 6 birthdays fall in M1 or M2 Thus the probability becomes 12C2 * 62/ 12^6 = 341/12^5 Yes, that's correct. Many of us have used binomial theorem extensively and know that: $$nC0 + nC1 + nC2 + ..... nCn = 2^n$$ (an application of binomial as described by you) Hence, it can be done this way. On the other hand, you could obviously say that each of the six people have 2 options - either his bday falls in J or in F. So those are a total of $$2*2*2*2*2*2 = 2^6$$ ways. Out of those, we need to remove 2 ways: All bdays fall in J and all bdays fall in F. Hence it can be done in $$(2^6 - 2)$$ ways. _________________ Karishma Veritas Prep | GMAT Instructor My Blog Get started with Veritas Prep GMAT On Demand for$199

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Re: Probability that the birthdays of 6 different persons will fall in exa [#permalink]

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29 Apr 2011, 06:07
@fluke, brilliant ! +1 for you
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Re: Probability that the birthdays of 6 different persons will fall in exa [#permalink]

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Re: Probability that the birthdays of 6 different persons will fall in exa [#permalink]

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05 Sep 2017, 00:15
Probability that the birthdays of 6 different persons will fall in exactly 2 calendar months is

A. 341/12^6
B. 352/12^6
C. 352/12^5
D. 341/12^5
E. 371/12^6

Really mind boggling question, You can't solve it in one go unless and until you have solved such question before..
fluke & VeritasPrepKarishma very nice solution...
It seems easy once we read the solution but it's a real tough question..
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Re: Probability that the birthdays of 6 different persons will fall in exa [#permalink]

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10 Sep 2017, 01:49
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Expert's post
VeritasPrepKarishma wrote:
Probability that the birthdays of 6 different persons will fall in exactly 2 calendar months is

A. 341/12^6
B. 352/12^6
C. 352/12^5
D. 341/12^5
E. 371/12^6

Another slightly different way of approaching this (and a little more complicated than fluke's - which I think is a very appropriate solution. Nevertheless, it might be useful for people who have done permutation combination before).
Select 2 months and then no of people for each month. You select two months in 12C2 ways. Say you get J and F.
Then you say that J could have 1 person's bday, 2 people's bday, 3 people's bday etc

J - 1/2/3/4/5 (You select 1 or 2 or 3 or 4 or 5 people for J)
F - 5/4/3/2/1 (Corresponding to no of people selected for J, remaining people go in F)

The distribution can be done in $$6C1 + 6C2 + 6C3 + 6C4 + 6C5 = 2^6 - 2 = 62$$ ways (If you recognize it, great, else just calculate)

So probability $$= 12C2 * 62/ 12^6 = 341/12^5$$

Responding to a pm:
Quote:

12C2 = 12*11/2

$$\frac{12C2 * 62}{12^6} = \frac{12*11*62}{2*12^6} = \frac{11*31}{12^5} = \frac{341}{12^5}$$
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Re: Probability that the birthdays of 6 different persons will fall in exa   [#permalink] 10 Sep 2017, 01:49
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