help with perms and coms (1 Viewer)

[ezzy85]

Does anyone know some hints or tricks for permutations and combinations? i can do the somple ones but as they get more complicated i just get more lost. how can you tell the difference when they ask for a P or a C? an example question:

The number of ways 5 boys and 5 girls sit around a table.

a. a particular girl must sit between 2 boys
b. 2 particular girls wish to sit directly opposite
c. 2 particular people do not want to sit together.

Also confused about the questions with trains where people have to sit in certain positions.
Mayve there are some sites to explain this?
Thanks for your help.

 

[McLake]

Does anyone know some hints or tricks for permutations and combinations? i can do the somple ones but as they get more complicated i just get more lost.

I know of no tricks, these questions are quite hard at the begining, but they will get easier ...


how can you tell the difference when they ask for a P or a C?

P is ordered selection, C is unordered selection.


The number of ways 5 boys and 5 girls sit around a table.

a. a particular girl must sit between 2 boys
b. 2 particular girls wish to sit directly opposite
c. 2 particular people do not want to sit together.


NB: Since there are 10 people, there are 10! combinations they can sit. But if the table looks the same from every direction then there are 10!/10 combos or 9! combinations.

a. place the girl "G" first, she can sit in ten spots. Now two boys must sit either side, so on her left their are 5 people possible, and on her right 4 people are possible. Seat the rest. 7 possible people in one of the remaing spots, then 6, etc ...

Here is equation: (10 * 5 * 4 * 7 * 6 * 5 * 4 * 3 * 2 * 1)/10


b. place girls first. girl "G1" can sit in 10 spots, now girl "G2" must sit opposite, ie. in a particular spot. Seat the rest. 8 possible, then 7, etc ...

Here is equation: (10 * 1 * 8 * 7 * 6 * 5 * 4 * 3 * 2 * 1)/10

c. place people first. "P1" can sit in 10 spots. "P2" can sit in 7 spots (can't sit either side or where "P1" is sitting). Seat the rest. 8 possible, then 7, etc ...

Here is equation: (10 * 7 * 8 * 7 * 6 * 5 * 4 * 3 * 2 * 1)/10



Also confused about the questions with trains where people have to sit in certain positions.

Please give an example question.



Hope this helps. Can anyone verify the answers for me???

 

[-=«MÄLÅÇhïtÊ»=-]

havent done maths for ages, but ye ur explainations seem correct

in terms of tips for these questions...do as many as possible so uno how to do em wen they pop up

dunno if this is a tip or not, but questions like:
"2 particular people do not want to sit together."
u can find the complemetary.
ie. find the possible ays of 2 ppl sitting 2gether and minus that from total ways possible wiv no restrictions

 

[ezzy85]

thanks

thanks alot for your help. appreciate it