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In how many ways can a teacher in a kindergarten school

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In how many ways can a teacher in a kindergarten school  [#permalink]

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New post 26 Jul 2012, 05:31
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In how many ways can a teacher in a kindergarten school arrange a group of 6 children (Susan, Karen, lei, Tim, joy and Zen) on 6 identical chairs in a straight line so that Susan is on the left of Tim?

A. 720
B. 360
C. 240
D. 120
E. 60
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Re: In how many ways can a teacher in a kindergarten school  [#permalink]

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New post 21 Dec 2015, 11:05
6
1
Vamshi8411 wrote:
In how many ways can a teacher in a kindergarten school arrange a group of 6 children (Susan, Karen, lei, Tim, joy and Zen) on 6 identical chairs in a straight line so that Susan is on the left of Tim?

A. 720
B. 360
C. 240
D. 120
E. 60


Method-1:

Total ways in which 6 children can be arranged on 6 chairs = 6*5*4*3*2*1 = 6! = 720

But in half cases Susan will be left of Tim and in other half of cases Tim will be on left of Susan

i.e. Desired cases in which Susan is on the left of Tim = (1/2)*720 = 360

Method-2:

We have six vacant chairs _ _ _ _ _ _

out of which 2 have to be occupied by Tim and Susan, No. of ways of selecting chairs for Tim and Susan = 6C2 = 15

No. of ways of arranging Tim an Susan on selected chairs = 1 (Because Susan must be on the left of Tim)

Total ways of arranging 4 remaining children on 4 remaining chairs = 4*3*2*1 = 4! = 24

i.e. Total Arrangement = 6C2 * 1 * 4! = 15*1*24 = 360

Method-3:

Case 1: S _ _ _ _ _
Total ways of arranging Tim to the right of Susan = 5
Total ways of arranging Remaining 4 children = 4!
i.e. Total Arrangement in this case = 5*4!

Case 2: _ S _ _ _ _
Total ways of arranging Tim to the right of Susan = 4
Total ways of arranging Remaining 4 children = 4!
i.e. Total Arrangement in this case = 4*4!

Case 3: _ _ S _ _ _
Total ways of arranging Tim to the right of Susan = 3
Total ways of arranging Remaining 4 children = 4!
i.e. Total Arrangement in this case = 3*4!

Case 4: _ _ _ S _ _
Total ways of arranging Tim to the right of Susan = 2
Total ways of arranging Remaining 4 children = 4!
i.e. Total Arrangement in this case = 2*4!

Case 5: _ _ _ _ S _
Total ways of arranging Tim to the right of Susan = 1
Total ways of arranging Remaining 4 children = 4!
i.e. Total Arrangement in this case = 1*4!

Total Arrangements = 5*4!+5*4!+4*4!+3*4!+2*4!+1*4! = 4!(5+4+3+2+1) = 24*15 = 360

Answer: Option B
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Re: In how many ways can a teacher in a kindergarten school  [#permalink]

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New post 26 Jul 2012, 05:43
2
3
Vamshi8411 wrote:
In how many ways can a teacher in a kindergarten school arrange a group of 6 children (Susan, Karen, lei, Tim, joy and Zen) on 6 identical chairs in a straight line so that Susan is on the left of Tim?

A. 720
B. 360
C. 240
D. 120
E. 60


6 children can be arranged in 6! ways. Now, in half of these arrangements Susan will be on the left of Tim and in half she'l be on the right of Tim, so the answer is 6!/2=360.

Answer: B.

Similar questions to practice:
susan-john-daisy-tim-matt-and-kim-need-to-be-seated-in-130743.html
goldenrod-and-no-hope-are-in-a-horse-race-with-6-contestants-82214.html
in-how-many-different-ways-can-the-letters-a-a-b-91460.html
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Re: In how many ways can a teacher in a kindergarten school  [#permalink]

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New post 26 Jul 2012, 08:18
Bunuel wrote:
Vamshi8411 wrote:
In how many ways can a teacher in a kindergarten school arrange a group of 6 children (Susan, Karen, lei, Tim, joy and Zen) on 6 identical chairs in a straight line so that Susan is on the left of Tim?

A. 720
B. 360
C. 240
D. 120
E. 60


6 children can be arranged in 6! ways. Now, in half of these arrangements Susan will be on the left of Tim and in half she'l be on the right of Tim, so the answer is 6!/2=360.

Answer: B.

Similar questions to practice:
susan-john-daisy-tim-matt-and-kim-need-to-be-seated-in-130743.html
goldenrod-and-no-hope-are-in-a-horse-race-with-6-contestants-82214.html
in-how-many-different-ways-can-the-letters-a-a-b-91460.html


Hi Bunuel,

can you please help me understand how did u come to the conclusion that for half the arrangements S is to the left of T and other half to the right of T?

Regards
Harsha
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Re: In how many ways can a teacher in a kindergarten school  [#permalink]

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New post 26 Jul 2012, 08:26
1
harshavmrg wrote:
Bunuel wrote:
Vamshi8411 wrote:
In how many ways can a teacher in a kindergarten school arrange a group of 6 children (Susan, Karen, lei, Tim, joy and Zen) on 6 identical chairs in a straight line so that Susan is on the left of Tim?

A. 720
B. 360
C. 240
D. 120
E. 60


6 children can be arranged in 6! ways. Now, in half of these arrangements Susan will be on the left of Tim and in half she'l be on the right of Tim, so the answer is 6!/2=360.

Answer: B.

Similar questions to practice:
susan-john-daisy-tim-matt-and-kim-need-to-be-seated-in-130743.html
goldenrod-and-no-hope-are-in-a-horse-race-with-6-contestants-82214.html
in-how-many-different-ways-can-the-letters-a-a-b-91460.html


Hi Bunuel,

can you please help me understand how did u come to the conclusion that for half the arrangements S is to the left of T and other half to the right of T?

Regards
Harsha


S and T can have ONLY TWO positions: S to the right of T OR to the left, how else?

Now, why should S be in more cases to the right of T than to the left? Does probability favors either of them? No. Hence exactly in half of these cases S will be to the right of T and in half of these cases to the left.

Check these links for more:
susan-john-daisy-tim-matt-and-kim-need-to-be-seated-in-130743.html
goldenrod-and-no-hope-are-in-a-horse-race-with-6-contestants-82214.html
in-how-many-different-ways-can-the-letters-a-a-b-91460.html
_________________

New to the Math Forum?
Please read this: Ultimate GMAT Quantitative Megathread | All You Need for Quant | PLEASE READ AND FOLLOW: 12 Rules for Posting!!!

Resources:
GMAT Math Book | Triangles | Polygons | Coordinate Geometry | Factorials | Circles | Number Theory | Remainders; 8. Overlapping Sets | PDF of Math Book; 10. Remainders | GMAT Prep Software Analysis | SEVEN SAMURAI OF 2012 (BEST DISCUSSIONS) | Tricky questions from previous years.

Collection of Questions:
PS: 1. Tough and Tricky questions; 2. Hard questions; 3. Hard questions part 2; 4. Standard deviation; 5. Tough Problem Solving Questions With Solutions; 6. Probability and Combinations Questions With Solutions; 7 Tough and tricky exponents and roots questions; 8 12 Easy Pieces (or not?); 9 Bakers' Dozen; 10 Algebra set. ,11 Mixed Questions, 12 Fresh Meat

DS: 1. DS tough questions; 2. DS tough questions part 2; 3. DS tough questions part 3; 4. DS Standard deviation; 5. Inequalities; 6. 700+ GMAT Data Sufficiency Questions With Explanations; 7 Tough and tricky exponents and roots questions; 8 The Discreet Charm of the DS; 9 Devil's Dozen!!!; 10 Number Properties set., 11 New DS set.


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Re: In how many ways can a teacher in a kindergarten school  [#permalink]

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New post 18 Aug 2017, 09:18
Vamshi8411 wrote:
In how many ways can a teacher in a kindergarten school arrange a group of 6 children (Susan, Karen, lei, Tim, joy and Zen) on 6 identical chairs in a straight line so that Susan is on the left of Tim?

A. 720
B. 360
C. 240
D. 120
E. 60


We see that the 6 students can be arranged in 6! = 720 ways. We also can determine that the number of ways in which Susan can be placed to the left of Tim is equal to the number of ways in which she can be placed to the right of Tim. Half the time she is to his right and half the time she is to his left. Thus, the number of ways the teacher can arrange the group with Susan to the left of Tim is 720/2 = 360 ways.

Answer: B
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Re: In how many ways can a teacher in a kindergarten school  [#permalink]

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New post 21 May 2018, 07:30
Note that if we read the question as number of ways where the two children are right next to one another, then the answer becomes 5!. Indeed assuming Tim is the first person to sit, he only has 5 spots, not 6, to choose from - he cannot sit at the far left.
So answer is (6-1)!
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Re: In how many ways can a teacher in a kindergarten school  [#permalink]

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New post 21 May 2018, 20:32
In how many ways can a teacher in a kindergarten school arrange a group of 6 children (Susan, Karen, lei, Tim, joy and Zen) on 6 identical chairs in a straight line so that Susan is on the left of Tim?

Total number of possibilities= 6! = 720

constraints:
We need to arrange 2 chairs for susan and tim so that susan is on the left of tim

Step 1:
Possibility of arranging 2 chairs for Susan and Tim = The possibilities is 6C2 =\(\frac{6!}{2!*4}\)! = 15

Step 2:
Number of arrangement for other 4 children = 4! = 24

Total number of arrangements = 15 * 24 = 360

Ans:B
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Re: In how many ways can a teacher in a kindergarten school &nbs [#permalink] 21 May 2018, 20:32
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