Perms and Combs. (1 Viewer)

[RivalryofTroll]

What do the answers say?

140

 

[Carrotsticks]

It's 8C5.

 

[theind1996]

140

Is there no working for the answers?

 

[theind1996]

Suppose A and B are in a circle, and DON'T want to sit next to each other.

Then we do Total - together = 9! - 8!2!

Sorry - I'm confused as to why it's this.

 

[RivalryofTroll]

Is there no working for the answers?

none. no working out.

8C5 should be right though... dam dodgy answers...

 

[theind1996]

none. no working out.

8C5 should be right though... dam dodgy answers...

Yea it should defs be 8C5.

 

[Carrotsticks]

Sorry - I'm confused as to why it's this.

XXXXX(AB)XXX

So we have 9 elements there (we count the AB as a group since they are together), but they are in a circle, so 8!.

Now A and B can swap, so 2!.

Therefore 8!2!.

 

[theind1996]

XXXXX(AB)XXX

So we have 9 elements there (we count the AB as a group since they are together), but they are in a circle, so 8!.

Now A and B can swap, so 2!.

Therefore 8!2!.

Oh - thanks a lot

Don't know why, but I always get confused with circles.

Thanks heaps

 

[RivalryofTroll]

Oh - thanks a lot

Don't know why, but I always get confused with circles.

Thanks heaps

well, isn't there really just one difference to 'line' questions? (i.e. one is excluded)

 

[RivalryofTroll]

Question: How do we know when to use the multiplication principle? Is there a certain way that the question is asked to hint us to use it?

 

[Carrotsticks]

Question: How do we know when to use the multiplication principle? Is there a certain way that the question is asked to hint us to use it?

If you're working on a single case and doing everything at once, then multiply.

 

[RivalryofTroll]

A meeting room contains a round table surrounded by ten chairs.
These chairs are indistinguishable and equally spaced around the table.

(a) A committe of 10 people incl. 3 teens. How many arrangements are there in which all 3 sit together? 30240

(b) Elections are held for the positions of Chairperson and Secretary in a 2nd committee of ten people seating around this table. How many seating arrangements are there in which two people elected are sitting directly opp. to each other?

 

[Deliriously]

A committee of 6 is to be selected from 10 people of whom A and B are two. How many committees can be formed excluding A if B is included?

10 people to choose to form 6.

For this question, there are two cases:
When B is included (A must be excluded) - 8 (b/c B has to be included and A excluded) to choose from 5 ( B is already in) .'. 8C5
When B is excluded (A doesn't have to be excluded)- 9 (B is excluded) to choose from 6 (anyone else can be chosen) .'. 9C6
= 8C5+9C6 = 140

 

[theind1996]

10 people to choose from 6.

For this question, there are two cases:
When B is included (A must be excluded) - 8 (b/c B has to be included and A excluded) to choose from 5 ( B is already in) .'. 8C5
When B is excluded (A doesn't have to be excluded)- 9 (B is excluded) to choose from 6 (anyone else can be chosen) .'. 9C6
= 8C5+9C6 = 140

Doesn't the question ask to include B?

 

[Deliriously]

Doesn't the question ask to include B?

Nope, it just says IF B is included , then A is excluded
but B can be excluded too.

 

[RivalryofTroll]

10 people to choose to form 6.

For this question, there are two cases:
When B is included (A must be excluded) - 8 (b/c B has to be included and A excluded) to choose from 5 ( B is already in) .'. 8C5
When B is excluded (A doesn't have to be excluded)- 9 (B is excluded) to choose from 6 (anyone else can be chosen) .'. 9C6
= 8C5+9C6 = 140

Thanks very much.

For my most recent question above, is it just 8! = Answer ?

 

[Carrotsticks]

I think it depends on how you read the question, and placement of commas.

How I interpreted it:

A committee of 6 is to be selected from 10 people of whom A and B are two. How many committees can be formed excluding A, if B is included?

What the question presumably tried to communicate

A committee of 6 is to be selected from 10 people of whom A and B are two. How many committees can be formed, excluding A if B is included?

 

[theind1996]

Nope, it just says IF B is included , then A is excluded
but B can be excluded too.

A committee of 6 is to be selected from 10 people of whom A and B are two. How many committees can be formed excluding A if B is included?

?

 

[Carrotsticks]

?

Refer to my post about placement of commas.

 

[theind1996]

I think it depends on how you read the question, and placement of commas.

How I interpreted it:

A committee of 6 is to be selected from 10 people of whom A and B are two. How many committees can be formed excluding A, if B is included?

What the question presumably tried to communicate

A committee of 6 is to be selected from 10 people of whom A and B are two. How many committees can be formed, excluding A if B is included?

English... haunts you to this day.