Easy probability Question O.L.. Please?

Raeone

Would anyone be so kind and explain to me how to do this question, I don't know how to do it

Helen has enough credit to download three songs from the internet, There are 7 songs that she wants.
(i) How many different possible selections of three songs can she make?
(ii) If there is one particular song that she wants, how many different selections now can she make??

I know its easy.. but my minds gone blank.

finality

i) 7C3

ii) she'll always have one particular song, meaning the choice is reduced to 6C2 x 1

WarZoneBrother

finality wrote: »

i) 7C3

ii) she'll always have one particular song, meaning the choice is reduced to 6C2 x 1

Yeah 7 Choose 3... Havent a notion how to do it on the calculator though ... Is there a smart way of doing this in your head?

Raeone

What do you mean 7C3?? what do you mean by the C? :P

Bears and Vodka

WarZoneBrother wrote: »

Yeah 7 Choose 3... Havent a notion how to do it on the calculator though ... Is there a smart way of doing this in your head?

On Sharp EL-W531 it's 7, [2nd F] 5, then 3. It gives the answer of 35.

Raeone wrote: »

What do you mean 7C3?? what do you mean by the C? :P

C stands for choose. 2nd F 5 on the Sharp calculator

Raeone

First time ever seeing that, thanks

WarZoneBrother

subz3r0 wrote: »

On Sharp EL-W531 it's 7, [2nd F] 5, then 3. It gives the answer of 35.




C stands for choose. 2nd F 5 on the Sharp calculator

I use a Casio.. But I will just ask teacher tomorrow going in at 9.30 to look at the stuff...

Naomi00

It think that's when you use the ! button?

Mayo_Boy

Do you get full marks for doing it on a calculator?

Raeone

Mayo_Boy wrote: »

Do you get full marks for doing it on a calculator?

Yes!!

Raeone

The first time I did this I did 7X6X5 = 210
Why don't I use that way?
How do I know when to use this C and not the first way?

ChemHickey

Raeone wrote: »

The first time I did this I did 7X6X5 = 210
Why don't I use that way?
How do I know when to use this C and not the first way?

If you want to do it out in your head- don't forget to divide by 3x2x1. as 7C3 is the same as 7! divided by 3!(7-3)! So it'd be (7x6x5x4x3x2x1) divided by (3x2x1x4x3x2x1) giving you 35 selections.

it's much faster to do it in your head if you're able!

finality

Raeone wrote: »

The first time I did this I did 7X6X5 = 210
Why don't I use that way?
How do I know when to use this C and not the first way?

That's the number of permutations, not the number of combinations. If you were asked how many ways could she arrange three out of seven songs, that's what you'd do.

Here's how to do combinations:



On a Casio calculator the nCr button is the ÷ button, that is, press shift and then ÷

Bears and Vodka

mind = blown

clairey__Ox

I'm 99% sure this isn't on our course, I remember not being able to do it in class and the teacher said we don't need to know how to do it!

Mayo_Boy

Im 99% sure that it is on our course :rolleyes:

Namlub

Your teacher's lying

Bears and Vodka

Can anybody tell me how to go about doing this?





Thank you.

grantyrs10

of course they are on the course !!!!!!!! ha no pun intended

NcR, sure it is mixed in with the Bernouilli Trials !!!!!

grantyrs10

oops im sorry ha this isnt on ordinary level as far as I know...
Ithought this was honours thread !

finality

subz3r0 wrote: »

Can anybody tell me how to go about doing this?

Thank you.

I think you'd divide the number of 4 person households in 2006 by the total number of households that year (that's the probability that a 4 person household is selected)

and then multiply by 1000

iluvgreenday246

Raeone wrote: »

Would anyone be so kind and explain to me how to do this question, I don't know how to do it

Helen has enough credit to download three songs from the internet, There are 7 songs that she wants.
(i) How many different possible selections of three songs can she make?
(ii) If there is one particular song that she wants, how many different selections now can she make??

I know its easy.. but my minds gone blank.

(i) 7x6x5=210 different selections

(ii) 1x6x5=30 different selections

finality

iluvgreenday246 wrote: »

(i) 7x6x5=210 different selections

(ii) 1x6x5=30 different selections

This is permutations, not combinations.

ChemHickey

iluvgreenday246 wrote: »

(i) 7x6x5=210 different selections

(ii) 1x6x5=30 different selections

If you say that, you're saying that if you downloaded song 1 before song 2 it matters, for this question you only care what song you want, not what order you want them/download them in!

ChemHickey

finality wrote: »

This is permutations, not combinations.

Finality, I don't think we should be snooping in this thread... we've a horror of a paper to worry about...

Raeone

Thanks a million everyone. It was from the 2010 paper I think so paper it isn't on the course anymore. I understand how to do it now though!!

finality

ChemHickey wrote: »

Finality, I don't think we should be snooping in this thread... we've a horror of a paper to worry about...

I know. I've been working at probability all day though. I wanted to take my mind off it...by posting about probability online.

ChemHickey

finality wrote: »

I know. I've been working at probability all day though. I wanted to take my mind off it...by posting about probability online.

I haven't done anything. I might do some now. Recommendations?

clairey__Ox

Raeone wrote: »

Thanks a million everyone. It was from the 2010 paper I think so paper it isn't on the course anymore. I understand how to do it now though!!

I promise you, it's not on the course!

clairey__Ox

Mayo_Boy wrote: »

Im 99% sure that it is on our course :rolleyes:

Namlub wrote: »

Your teacher's lying

Nope, it's not on it. It was on the 2010 sample paper and for non pilot schools it is not on the course this year.
I can't speak for pilot schools but for the majority it's not on tomorrows paper, so I'm not lying :rolleyes:

Best of luck tomorrrow!

Raeone

Thanks

finality

It's definitely not on the OL course, I noticed that in one of the SEC circular letters. So no need to worry about this guys

Namlub

If it's seriously not on the course I don't know why I'm even bothering with sample papers because they obviously don't correspond to the syllabus, ugh.

whiteandlight

Which circular was that? You all seem to mixing up permutations and combinations. Permutations are on the HL course and not the ordinary but combinations are on both.

Combinations are ways of choosing r objects from n objects when you don't care about order eg when downloading songs, it doesn't matter which you download first.

Permutations are ways of choosing r objects from n objects when order matters eg when choosing first, second and third place in a competition.

Only combinations are on OL and you can do them using the nCr function on your calculator.

Eg how many committees of three can be chosen from seven people?
7ncr3

If one of the seven members John can not be chosen, how many committees are possible?
There are only 6 people left to chose from but we still need three
6ncr3

If Mary has to be on the committee how many possible arrangements are there?
Mary is chosen leaving 6 people to pick from but we already have one of the three positions filled so we only need two more
6ncr2