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# Two Part Analysis - Combinatorics Question

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Senior Manager
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Two Part Analysis - Combinatorics Question [#permalink]  25 Aug 2012, 07:18
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A sports coach intends to choose a team of players from a pool of candidates. The coach wants to be able to have more than 20 but fewer than 25 distinct possibilities for the composition of the chosen team, with at least as many candidates chosen for the team as those not chosen.

Identify the number of candidates in the pool and the number of players on the team that are consistent with the coach’s intentions.
Make only two selections, one in each column.

Number | Candidates in Pool | Players on Team
3
4
5
6
7
8

Solutions:
[Reveal] Spoiler:
Candidates in Pool: 7
Players on Team: 5
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Re: Two Part Analysis - Combinatorics Question [#permalink]  25 Aug 2012, 09:27
macjas wrote:
A sports coach intends to choose a team of players from a pool of candidates. The coach wants to be able to have more than 20 but fewer than 25 distinct possibilities for the composition of the chosen team, with at least as many candidates chosen for the team as those not chosen.

Identify the number of candidates in the pool and the number of players on the team that are consistent with the coach’s intentions.
Make only two selections, one in each column.

Number | Candidates in Pool | Players on Team
3
4
5
6
7
8

Solutions:
[Reveal] Spoiler:
Candidates in Pool: 7
Players on Team: 5

For a given number of candidates, we can list the values for each number of choices. A useful tool in this case is the so-called Pascal's triangle (see for example http://www.mathwords.com/b/binomial_coe ... pascal.htm).

1
1 1
1 2 1
1 3 3 1
1 4 6 4 1
1 5 10 10 5 1
1 6 15 20 15 6 1
1 7 21 35 35 21 7 1
1 8 18 56 70 56 28 1

We can see that n = 7 as the number of candidates and k = 5 as the number of chosen players fulfill the requirements.
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Senior Manager
Affiliations: UWC
Joined: 09 May 2012
Posts: 399
GMAT 1: 620 Q42 V33
GMAT 2: 680 Q44 V38
GPA: 3.43
WE: Engineering (Entertainment and Sports)
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Re: Two Part Analysis - Combinatorics Question [#permalink]  26 Aug 2012, 00:35
Hi there, this is an interesting approach.
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Re: Two Part Analysis - Combinatorics Question [#permalink]  26 Aug 2012, 10:20
EvaJager wrote:
macjas wrote:
A sports coach intends to choose a team of players from a pool of candidates. The coach wants to be able to have more than 20 but fewer than 25 distinct possibilities for the composition of the chosen team, with at least as many candidates chosen for the team as those not chosen.

Identify the number of candidates in the pool and the number of players on the team that are consistent with the coach’s intentions.
Make only two selections, one in each column.

Number | Candidates in Pool | Players on Team
3
4
5
6
7
8

Solutions:
[Reveal] Spoiler:
Candidates in Pool: 7
Players on Team: 5

For a given number of candidates, we can list the values for each number of choices. A useful tool in this case is the so-called Pascal's triangle (see for example http://www.mathwords.com/b/binomial_coe ... pascal.htm).

1
1 1
1 2 1
1 3 3 1
1 4 6 4 1
1 5 10 10 5 1
1 6 15 20 15 6 1
1 7 21 35 35 21 7 1
1 8 18 56 70 56 28 1

We can see that n = 7 as the number of candidates and k = 5 as the number of chosen players fulfill the requirements.

Is the question not saying $$nCr = nCn-r$$ and find value of n and r for which $$25>nCr>20$$ ??
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Re: Two Part Analysis - Combinatorics Question [#permalink]  26 Aug 2012, 10:50
mandyrhtdm wrote:
EvaJager wrote:
macjas wrote:
A sports coach intends to choose a team of players from a pool of candidates. The coach wants to be able to have more than 20 but fewer than 25 distinct possibilities for the composition of the chosen team, with at least as many candidates chosen for the team as those not chosen.

Identify the number of candidates in the pool and the number of players on the team that are consistent with the coach’s intentions.
Make only two selections, one in each column.

Number | Candidates in Pool | Players on Team
3
4
5
6
7
8

Solutions:
[Reveal] Spoiler:
Candidates in Pool: 7
Players on Team: 5

For a given number of candidates, we can list the values for each number of choices. A useful tool in this case is the so-called Pascal's triangle (see for example http://www.mathwords.com/b/binomial_coe ... pascal.htm).

1
1 1
1 2 1
1 3 3 1
1 4 6 4 1
1 5 10 10 5 1
1 6 15 20 15 6 1
1 7 21 35 35 21 7 1
1 8 18 56 70 56 28 1

We can see that n = 7 as the number of candidates and k = 5 as the number of chosen players fulfill the requirements.

Is the question not saying $$nCr = nCn-r$$ and find value of n and r for which $$25>nCr>20$$ ??

The question says "with at least as many candidates chosen for the team as those not chosen" and in our case chosen is $$5 \geq 2$$ which are not chosen.
In addition, 7C5 = 7C2 = 7*6/2 = 21 > 20 and 21 < 25.
And for n = 8, there is no k fulfilling the required conditions.
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Re: Two Part Analysis - Combinatorics Question [#permalink]  28 Aug 2012, 09:50
is there any other approach to solving this problem?
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Re: Two Part Analysis - Combinatorics Question [#permalink]  28 Aug 2012, 10:25
1
KUDOS
abheeeee wrote:
is there any other approach to solving this problem?

nCk = nC(n - k) is the easiest to compute for k = 2 being equal to $$\frac{n(n-1)}{2}.$$
You should start with the highest values and go backwards, as 6C3 = 20 and nCk is the highest for k around half of n.
So, you can deduce that n must be greater than 6.
And take advantage of the multiple choice question, as there is just one correct answer.

Otherwise, once you understand Pascal's triangle, it is quite easy to write it down. Having all the values in front of yours eyes, really helps to pinpoint the correct answer.
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Re: Two Part Analysis - Combinatorics Question [#permalink]  06 Sep 2012, 18:07
I think we have to pick numbers many times until we find the answer. We can not remember any formular because gmat dose not requires us to do so.
a long time and patience are required.
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Re: Two Part Analysis - Combinatorics Question [#permalink]  27 Sep 2012, 08:13
For choosing teams, you want to keep in mind the combination basics...
you want to choose out of n people, create a team of r. THe number of combinations (NOT permutations) should be between 21-24. That's more than 20 and less than 25.

So, if you try the various combinations the only one that works is out of 7 choose a team of 5.

THat's 7NCR5
7! / (5! 2!) = 6*7/2 = 21

8! / (4!*4!) = 5*6*7*8/4 = 30*7*2 = 240

8! / (5! 3!) = 6*7*8 / (3*2) = 56

8! / (6! 2!) = 7*8 / 2 = 28

8! / (7! 1!) = 8

So somehow we don't get in the correct range. You can try for the others but you won't get in range.

6C3 = 6! / (3! 3!) = 4*5*6 / (3*2) = 20
6C4 = 6! / (4! 2!) = 5*6 / 2 = 15

The highest number of combinations will be in the middle. So 6C3 will be the max for 6 people. 4C2 will be the max for 4 people. So if 6C3 only has 20, we know we need to go higher to 7.

7C4 = 7! / (4! 3!) = 5*6*7 / (3*2) = 5*7 = 35 (too high)
7C5 = 7! / (5! 2) = 6*7 / 2 = 21 (just right!)

So if you're familiar with the binomial distribution curve for these combinations and that out of 8 you choose a number in the middle you'll get the highest number of results. Using that you can do an educated guess as to what to try next.
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Re: Two Part Analysis - Combinatorics Question [#permalink]  05 Oct 2012, 21:29
gmatpill wrote:
For choosing teams, you want to keep in mind the combination basics...
you want to choose out of n people, create a team of r. THe number of combinations (NOT permutations) should be between 21-24. That's more than 20 and less than 25.

So, if you try the various combinations the only one that works is out of 7 choose a team of 5.

THat's 7NCR5
7! / (5! 2!) = 6*7/2 = 21

8! / (4!*4!) = 5*6*7*8/4 = 30*7*2 = 240

8! / (5! 3!) = 6*7*8 / (3*2) = 56

8! / (6! 2!) = 7*8 / 2 = 28

8! / (7! 1!) = 8

So somehow we don't get in the correct range. You can try for the others but you won't get in range.

6C3 = 6! / (3! 3!) = 4*5*6 / (3*2) = 20
6C4 = 6! / (4! 2!) = 5*6 / 2 = 15

The highest number of combinations will be in the middle. So 6C3 will be the max for 6 people. 4C2 will be the max for 4 people. So if 6C3 only has 20, we know we need to go higher to 7.

7C4 = 7! / (4! 3!) = 5*6*7 / (3*2) = 5*7 = 35 (too high)
7C5 = 7! / (5! 2) = 6*7 / 2 = 21 (just right!)

So if you're familiar with the binomial distribution curve for these combinations and that out of 8 you choose a number in the middle you'll get the highest number of results. Using that you can do an educated guess as to what to try next.

But what does the question mean by"with at least as many candidates chosen for the team as those not chosen".
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Re: Two Part Analysis - Combinatorics Question [#permalink]  09 Oct 2012, 23:48
this is hard. I do not think we need remember the formular.
Just "feel" about correct answer and pick specific number to try. it take me long to do so.
Re: Two Part Analysis - Combinatorics Question   [#permalink] 09 Oct 2012, 23:48
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# Two Part Analysis - Combinatorics Question

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