Probability Tips (1 Viewer)

[Rahul]

any?

i tend to get a bit confused when dealing with probability questions. it is due to confusion as to when (n C k) or (n P k) apply. i tend to do it the 'manual' way, thus making errors.

can someone point out either when to apply either/both or 'definitions' of permutations and combinations.

any other tips associated with topic will also be helpful.

 

[Lazarus]

(n P k) = when the order that the objects are in matters

(n C k) = when only the set of objects chosen matters

 

[flyin']

[A side note]

If you do decide to do Actuarial, there'll be alot of Probability, you even get to spend a week doin' Permuations and week doin' Combinations (expect at a harder level).

 

[ND]

Originally posted by flyin'
[A side note]

If you do decide to do Actuarial, there'll be alot of Probability, you even get to spend a week doin' Permuations and week doin' Combinations (expect at a harder level).

Haha fun, probability just happens to be my worst (by far) subject.

 

[flyin']

Well, that's usually the case! (Unless you do practically every example in the Cambridge Ext1 textbook, and try to understand it. )

 

[Rahul]

i have been doing probability excercise recently, and getting the answers most of the time actually. yay for me

Just, i feel i dont follow the right procedures sometimes, when a question maybe a simple one regarding, (n C k), i will be confused. although this was actually a week ago, before i had done any of the excercises in cambridge.

hm...this thread is a nervous reaction i spose.

i actually like doing probability, just need to spend more time getting better at it.

permutations, are when you choose from a set and arrange them. eg- diff possibilities of lights(red, amber, green)
combinations, are when you choose a sub-set from a set. they arent/cant be arranged. eg- choosing 5 guys and 3 girls out of a bunch of people, etc.
that sort of thing?

 

[flyin']

Combinations are sets where order is not important.
Permutations are sets where order is important.

 

[0ver97]

how come there seems to be more combinations in a certain question then there are permutations. if indeed combs are in no certain order

like
n
C
5

is more than
n
P
5

 

[flyin']

There are fewer combinations (than permutations). Because for each combination, you can rearrange the items in the given set.

 

[0ver97]

its hard to get my head around, but i thought comb (because it doesnt matter its order) can take all the values of a perm and more

i just dont get it

 

[kimmeh]

hmm i agree
i just dont get it.. probability and permutations combinations.. i wish someone would just sit down with me and teach me it propely

 

[flyin']

Consider
1: {ABC}
2: {ACB}
3: {BCA}
4: {BAC}
5: {CBA}
6: {CAB}

Now 1 to 6 is just 1 combination. (Order doesn't matter.)
Whereas 1 to 6 is 6 permuations. (Order matters.)

Any clearer?

 

[Rahul]

another, slightly different example:

{A, B, C, D}

there will be (4 C 3) combinations of 3 letters.
4, imagine for each group of 3, one letter will be 'left out'. 4 letters can be left out, therefore 4 combinations.

there will be (4 P 3) permutations of 3 letters.
24, in the 4 combinations above, each of the letters in the 4 groups can be ordered 3! times. therefore, 4 x 3! = 24.

 

[flyin']

Yep, that's absolutely right! So things are cleared up.

 

[Rahul]

i think so.

cant wait to get into some of those questions