How many different arrangements are there if Bob and Sally must always be seated next to each other?
$begingroup$
How many different arrangements are there in which Bob, Sally and $n$ other people sit down in a row of $n+3$ chairs if Bob and Sally must always be seated next to each other?
I tried putting Bob and Sally next to each other in the first two chairs so then there are $2 times n!$ arrangements but then I need to move them to the 2nd and 3rd chair and so on. Not exactly sure how to do this.
combinatorics permutations
$endgroup$
add a comment |
$begingroup$
How many different arrangements are there in which Bob, Sally and $n$ other people sit down in a row of $n+3$ chairs if Bob and Sally must always be seated next to each other?
I tried putting Bob and Sally next to each other in the first two chairs so then there are $2 times n!$ arrangements but then I need to move them to the 2nd and 3rd chair and so on. Not exactly sure how to do this.
combinatorics permutations
$endgroup$
3
$begingroup$
What have you tried? Where are you stuck?
$endgroup$
– Arthur
Dec 7 '18 at 9:43
$begingroup$
I tired putting bob and sally next to each other in the first two chairs so then there is 2x(n!) arrangements but then I need to move them to the 2nd and 3rd chair and so on. Not exactly sure how to do this
$endgroup$
– user607735
Dec 7 '18 at 9:45
1
$begingroup$
Why not try to get an answer for $n = 0, 1$ or $2$ first to get a feel for what's going on?
$endgroup$
– Arthur
Dec 7 '18 at 9:47
$begingroup$
Is the problem stated correctly, so one seat remains empty?
$endgroup$
– Christoph
Dec 7 '18 at 9:53
$begingroup$
Please read this tutorial on how to typeset mathematics on this site.
$endgroup$
– N. F. Taussig
Dec 7 '18 at 10:07
add a comment |
$begingroup$
How many different arrangements are there in which Bob, Sally and $n$ other people sit down in a row of $n+3$ chairs if Bob and Sally must always be seated next to each other?
I tried putting Bob and Sally next to each other in the first two chairs so then there are $2 times n!$ arrangements but then I need to move them to the 2nd and 3rd chair and so on. Not exactly sure how to do this.
combinatorics permutations
$endgroup$
How many different arrangements are there in which Bob, Sally and $n$ other people sit down in a row of $n+3$ chairs if Bob and Sally must always be seated next to each other?
I tried putting Bob and Sally next to each other in the first two chairs so then there are $2 times n!$ arrangements but then I need to move them to the 2nd and 3rd chair and so on. Not exactly sure how to do this.
combinatorics permutations
combinatorics permutations
edited Dec 7 '18 at 10:07
N. F. Taussig
44.1k93356
44.1k93356
asked Dec 7 '18 at 9:39
user607735user607735
103
103
3
$begingroup$
What have you tried? Where are you stuck?
$endgroup$
– Arthur
Dec 7 '18 at 9:43
$begingroup$
I tired putting bob and sally next to each other in the first two chairs so then there is 2x(n!) arrangements but then I need to move them to the 2nd and 3rd chair and so on. Not exactly sure how to do this
$endgroup$
– user607735
Dec 7 '18 at 9:45
1
$begingroup$
Why not try to get an answer for $n = 0, 1$ or $2$ first to get a feel for what's going on?
$endgroup$
– Arthur
Dec 7 '18 at 9:47
$begingroup$
Is the problem stated correctly, so one seat remains empty?
$endgroup$
– Christoph
Dec 7 '18 at 9:53
$begingroup$
Please read this tutorial on how to typeset mathematics on this site.
$endgroup$
– N. F. Taussig
Dec 7 '18 at 10:07
add a comment |
3
$begingroup$
What have you tried? Where are you stuck?
$endgroup$
– Arthur
Dec 7 '18 at 9:43
$begingroup$
I tired putting bob and sally next to each other in the first two chairs so then there is 2x(n!) arrangements but then I need to move them to the 2nd and 3rd chair and so on. Not exactly sure how to do this
$endgroup$
– user607735
Dec 7 '18 at 9:45
1
$begingroup$
Why not try to get an answer for $n = 0, 1$ or $2$ first to get a feel for what's going on?
$endgroup$
– Arthur
Dec 7 '18 at 9:47
$begingroup$
Is the problem stated correctly, so one seat remains empty?
$endgroup$
– Christoph
Dec 7 '18 at 9:53
$begingroup$
Please read this tutorial on how to typeset mathematics on this site.
$endgroup$
– N. F. Taussig
Dec 7 '18 at 10:07
3
3
$begingroup$
What have you tried? Where are you stuck?
$endgroup$
– Arthur
Dec 7 '18 at 9:43
$begingroup$
What have you tried? Where are you stuck?
$endgroup$
– Arthur
Dec 7 '18 at 9:43
$begingroup$
I tired putting bob and sally next to each other in the first two chairs so then there is 2x(n!) arrangements but then I need to move them to the 2nd and 3rd chair and so on. Not exactly sure how to do this
$endgroup$
– user607735
Dec 7 '18 at 9:45
$begingroup$
I tired putting bob and sally next to each other in the first two chairs so then there is 2x(n!) arrangements but then I need to move them to the 2nd and 3rd chair and so on. Not exactly sure how to do this
$endgroup$
– user607735
Dec 7 '18 at 9:45
1
1
$begingroup$
Why not try to get an answer for $n = 0, 1$ or $2$ first to get a feel for what's going on?
$endgroup$
– Arthur
Dec 7 '18 at 9:47
$begingroup$
Why not try to get an answer for $n = 0, 1$ or $2$ first to get a feel for what's going on?
$endgroup$
– Arthur
Dec 7 '18 at 9:47
$begingroup$
Is the problem stated correctly, so one seat remains empty?
$endgroup$
– Christoph
Dec 7 '18 at 9:53
$begingroup$
Is the problem stated correctly, so one seat remains empty?
$endgroup$
– Christoph
Dec 7 '18 at 9:53
$begingroup$
Please read this tutorial on how to typeset mathematics on this site.
$endgroup$
– N. F. Taussig
Dec 7 '18 at 10:07
$begingroup$
Please read this tutorial on how to typeset mathematics on this site.
$endgroup$
– N. F. Taussig
Dec 7 '18 at 10:07
add a comment |
3 Answers
3
active
oldest
votes
$begingroup$
Hint: Consider Bob and Sally one unit taking two seats. So you have to place $n+1$ units at $n+2$ spots. In the end multiply by $2$ to account for Bob sitting on the left or right of Sally.
$endgroup$
$begingroup$
yes so it could be writing as n+2 C n+1
$endgroup$
– user607735
Dec 7 '18 at 9:55
1
$begingroup$
Writing what as $binom{n+2}{n+1}$? The number of ways to seat the people is something else.
$endgroup$
– Christoph
Dec 7 '18 at 9:56
add a comment |
$begingroup$
Treat Bob and Sally as the same person (say, V) that takes up $2$ spaces, i.e. you put it in chair $m$ and then you can't put anyone in chair $m+1$. This means V cannot be in chair $n+3$. So, we place $V$ first in $n+2$ seats, and since it takes up $2$ seats, we now have $n+1$ seats for $n$ people. This means that the number of permutations is $(n+2)!$. Now, we can have $2$ "states" for $V$: one where Bob sits on chair $m$ and Sally on chair $m+1$, or Sally sits on chair $m$ and Bob sits on $m+1$. So, we multiply by $2$ to account, for a grand total of $2(n+2)!$ seatings arrangements.
$endgroup$
add a comment |
$begingroup$
You can evaluate the arrangements separately.
So for instance, the different arrangements in which Bob and Sally sit together are $$(n+2)times 2$$
Note that deciding whether Bob or Sally sits to the right doubles the possibilities.
Now, all possible arrangements in which $n$ different people sit (it doesn't matter whether together or not) is simply $$n!$$
All possibilities are hence $$n!times (n+2)times2$$
Note, however, that you also have to consider the gap; the total amount of possibilities is thus $$n!times (n+2)times 2times(n+1)=2times(n+2)!$$
$endgroup$
$begingroup$
You could also just exchange the gap by an additional person (since there is only one) and obtain $(n+1)!$ instead of $n!$ immediately.
$endgroup$
– Christoph
Dec 7 '18 at 10:00
$begingroup$
@Christoph you're definitely right!
$endgroup$
– Dr. Mathva
Dec 7 '18 at 10:26
add a comment |
Your Answer
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3 Answers
3
active
oldest
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3 Answers
3
active
oldest
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active
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votes
$begingroup$
Hint: Consider Bob and Sally one unit taking two seats. So you have to place $n+1$ units at $n+2$ spots. In the end multiply by $2$ to account for Bob sitting on the left or right of Sally.
$endgroup$
$begingroup$
yes so it could be writing as n+2 C n+1
$endgroup$
– user607735
Dec 7 '18 at 9:55
1
$begingroup$
Writing what as $binom{n+2}{n+1}$? The number of ways to seat the people is something else.
$endgroup$
– Christoph
Dec 7 '18 at 9:56
add a comment |
$begingroup$
Hint: Consider Bob and Sally one unit taking two seats. So you have to place $n+1$ units at $n+2$ spots. In the end multiply by $2$ to account for Bob sitting on the left or right of Sally.
$endgroup$
$begingroup$
yes so it could be writing as n+2 C n+1
$endgroup$
– user607735
Dec 7 '18 at 9:55
1
$begingroup$
Writing what as $binom{n+2}{n+1}$? The number of ways to seat the people is something else.
$endgroup$
– Christoph
Dec 7 '18 at 9:56
add a comment |
$begingroup$
Hint: Consider Bob and Sally one unit taking two seats. So you have to place $n+1$ units at $n+2$ spots. In the end multiply by $2$ to account for Bob sitting on the left or right of Sally.
$endgroup$
Hint: Consider Bob and Sally one unit taking two seats. So you have to place $n+1$ units at $n+2$ spots. In the end multiply by $2$ to account for Bob sitting on the left or right of Sally.
edited Dec 7 '18 at 10:01
answered Dec 7 '18 at 9:52
ChristophChristoph
12k1642
12k1642
$begingroup$
yes so it could be writing as n+2 C n+1
$endgroup$
– user607735
Dec 7 '18 at 9:55
1
$begingroup$
Writing what as $binom{n+2}{n+1}$? The number of ways to seat the people is something else.
$endgroup$
– Christoph
Dec 7 '18 at 9:56
add a comment |
$begingroup$
yes so it could be writing as n+2 C n+1
$endgroup$
– user607735
Dec 7 '18 at 9:55
1
$begingroup$
Writing what as $binom{n+2}{n+1}$? The number of ways to seat the people is something else.
$endgroup$
– Christoph
Dec 7 '18 at 9:56
$begingroup$
yes so it could be writing as n+2 C n+1
$endgroup$
– user607735
Dec 7 '18 at 9:55
$begingroup$
yes so it could be writing as n+2 C n+1
$endgroup$
– user607735
Dec 7 '18 at 9:55
1
1
$begingroup$
Writing what as $binom{n+2}{n+1}$? The number of ways to seat the people is something else.
$endgroup$
– Christoph
Dec 7 '18 at 9:56
$begingroup$
Writing what as $binom{n+2}{n+1}$? The number of ways to seat the people is something else.
$endgroup$
– Christoph
Dec 7 '18 at 9:56
add a comment |
$begingroup$
Treat Bob and Sally as the same person (say, V) that takes up $2$ spaces, i.e. you put it in chair $m$ and then you can't put anyone in chair $m+1$. This means V cannot be in chair $n+3$. So, we place $V$ first in $n+2$ seats, and since it takes up $2$ seats, we now have $n+1$ seats for $n$ people. This means that the number of permutations is $(n+2)!$. Now, we can have $2$ "states" for $V$: one where Bob sits on chair $m$ and Sally on chair $m+1$, or Sally sits on chair $m$ and Bob sits on $m+1$. So, we multiply by $2$ to account, for a grand total of $2(n+2)!$ seatings arrangements.
$endgroup$
add a comment |
$begingroup$
Treat Bob and Sally as the same person (say, V) that takes up $2$ spaces, i.e. you put it in chair $m$ and then you can't put anyone in chair $m+1$. This means V cannot be in chair $n+3$. So, we place $V$ first in $n+2$ seats, and since it takes up $2$ seats, we now have $n+1$ seats for $n$ people. This means that the number of permutations is $(n+2)!$. Now, we can have $2$ "states" for $V$: one where Bob sits on chair $m$ and Sally on chair $m+1$, or Sally sits on chair $m$ and Bob sits on $m+1$. So, we multiply by $2$ to account, for a grand total of $2(n+2)!$ seatings arrangements.
$endgroup$
add a comment |
$begingroup$
Treat Bob and Sally as the same person (say, V) that takes up $2$ spaces, i.e. you put it in chair $m$ and then you can't put anyone in chair $m+1$. This means V cannot be in chair $n+3$. So, we place $V$ first in $n+2$ seats, and since it takes up $2$ seats, we now have $n+1$ seats for $n$ people. This means that the number of permutations is $(n+2)!$. Now, we can have $2$ "states" for $V$: one where Bob sits on chair $m$ and Sally on chair $m+1$, or Sally sits on chair $m$ and Bob sits on $m+1$. So, we multiply by $2$ to account, for a grand total of $2(n+2)!$ seatings arrangements.
$endgroup$
Treat Bob and Sally as the same person (say, V) that takes up $2$ spaces, i.e. you put it in chair $m$ and then you can't put anyone in chair $m+1$. This means V cannot be in chair $n+3$. So, we place $V$ first in $n+2$ seats, and since it takes up $2$ seats, we now have $n+1$ seats for $n$ people. This means that the number of permutations is $(n+2)!$. Now, we can have $2$ "states" for $V$: one where Bob sits on chair $m$ and Sally on chair $m+1$, or Sally sits on chair $m$ and Bob sits on $m+1$. So, we multiply by $2$ to account, for a grand total of $2(n+2)!$ seatings arrangements.
answered Dec 7 '18 at 9:56
KykyKyky
460213
460213
add a comment |
add a comment |
$begingroup$
You can evaluate the arrangements separately.
So for instance, the different arrangements in which Bob and Sally sit together are $$(n+2)times 2$$
Note that deciding whether Bob or Sally sits to the right doubles the possibilities.
Now, all possible arrangements in which $n$ different people sit (it doesn't matter whether together or not) is simply $$n!$$
All possibilities are hence $$n!times (n+2)times2$$
Note, however, that you also have to consider the gap; the total amount of possibilities is thus $$n!times (n+2)times 2times(n+1)=2times(n+2)!$$
$endgroup$
$begingroup$
You could also just exchange the gap by an additional person (since there is only one) and obtain $(n+1)!$ instead of $n!$ immediately.
$endgroup$
– Christoph
Dec 7 '18 at 10:00
$begingroup$
@Christoph you're definitely right!
$endgroup$
– Dr. Mathva
Dec 7 '18 at 10:26
add a comment |
$begingroup$
You can evaluate the arrangements separately.
So for instance, the different arrangements in which Bob and Sally sit together are $$(n+2)times 2$$
Note that deciding whether Bob or Sally sits to the right doubles the possibilities.
Now, all possible arrangements in which $n$ different people sit (it doesn't matter whether together or not) is simply $$n!$$
All possibilities are hence $$n!times (n+2)times2$$
Note, however, that you also have to consider the gap; the total amount of possibilities is thus $$n!times (n+2)times 2times(n+1)=2times(n+2)!$$
$endgroup$
$begingroup$
You could also just exchange the gap by an additional person (since there is only one) and obtain $(n+1)!$ instead of $n!$ immediately.
$endgroup$
– Christoph
Dec 7 '18 at 10:00
$begingroup$
@Christoph you're definitely right!
$endgroup$
– Dr. Mathva
Dec 7 '18 at 10:26
add a comment |
$begingroup$
You can evaluate the arrangements separately.
So for instance, the different arrangements in which Bob and Sally sit together are $$(n+2)times 2$$
Note that deciding whether Bob or Sally sits to the right doubles the possibilities.
Now, all possible arrangements in which $n$ different people sit (it doesn't matter whether together or not) is simply $$n!$$
All possibilities are hence $$n!times (n+2)times2$$
Note, however, that you also have to consider the gap; the total amount of possibilities is thus $$n!times (n+2)times 2times(n+1)=2times(n+2)!$$
$endgroup$
You can evaluate the arrangements separately.
So for instance, the different arrangements in which Bob and Sally sit together are $$(n+2)times 2$$
Note that deciding whether Bob or Sally sits to the right doubles the possibilities.
Now, all possible arrangements in which $n$ different people sit (it doesn't matter whether together or not) is simply $$n!$$
All possibilities are hence $$n!times (n+2)times2$$
Note, however, that you also have to consider the gap; the total amount of possibilities is thus $$n!times (n+2)times 2times(n+1)=2times(n+2)!$$
answered Dec 7 '18 at 9:58
Dr. MathvaDr. Mathva
1,102317
1,102317
$begingroup$
You could also just exchange the gap by an additional person (since there is only one) and obtain $(n+1)!$ instead of $n!$ immediately.
$endgroup$
– Christoph
Dec 7 '18 at 10:00
$begingroup$
@Christoph you're definitely right!
$endgroup$
– Dr. Mathva
Dec 7 '18 at 10:26
add a comment |
$begingroup$
You could also just exchange the gap by an additional person (since there is only one) and obtain $(n+1)!$ instead of $n!$ immediately.
$endgroup$
– Christoph
Dec 7 '18 at 10:00
$begingroup$
@Christoph you're definitely right!
$endgroup$
– Dr. Mathva
Dec 7 '18 at 10:26
$begingroup$
You could also just exchange the gap by an additional person (since there is only one) and obtain $(n+1)!$ instead of $n!$ immediately.
$endgroup$
– Christoph
Dec 7 '18 at 10:00
$begingroup$
You could also just exchange the gap by an additional person (since there is only one) and obtain $(n+1)!$ instead of $n!$ immediately.
$endgroup$
– Christoph
Dec 7 '18 at 10:00
$begingroup$
@Christoph you're definitely right!
$endgroup$
– Dr. Mathva
Dec 7 '18 at 10:26
$begingroup$
@Christoph you're definitely right!
$endgroup$
– Dr. Mathva
Dec 7 '18 at 10:26
add a comment |
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3
$begingroup$
What have you tried? Where are you stuck?
$endgroup$
– Arthur
Dec 7 '18 at 9:43
$begingroup$
I tired putting bob and sally next to each other in the first two chairs so then there is 2x(n!) arrangements but then I need to move them to the 2nd and 3rd chair and so on. Not exactly sure how to do this
$endgroup$
– user607735
Dec 7 '18 at 9:45
1
$begingroup$
Why not try to get an answer for $n = 0, 1$ or $2$ first to get a feel for what's going on?
$endgroup$
– Arthur
Dec 7 '18 at 9:47
$begingroup$
Is the problem stated correctly, so one seat remains empty?
$endgroup$
– Christoph
Dec 7 '18 at 9:53
$begingroup$
Please read this tutorial on how to typeset mathematics on this site.
$endgroup$
– N. F. Taussig
Dec 7 '18 at 10:07