# probability killer question!!!!!!!!!!!!!!!!!!!!!!!!!!!! (1 Viewer)

#### ww0811

##### Member
An urn contains 10 blue balls, 5 green balls and 5 red balls. if 3 balls are drawn randomly without replacement, given that the first two balls are not red, what is the probability that the third ball is green?

#### Speed6

##### Retired '16
Man, probability always gets me aswell.

#### callipygian

##### Member

I'd work out the probability of each possibility - you can either draw BBG, BGG, GBG, or GGG - and add them together. (Could be wrong hahaha, probability was always my weakest topic)

#### Queenroot

##### I complete the Squar3
not sure if square just seenzoned this or hasn't seen it

#### Speed6

##### Retired '16
not sure if square just seenzoned this or hasn't seen it
Probably seenzoned it and said this topic got me in the HSC.

Thus, saying gg no re.

#### BLIT2014

##### The pessimistic optimist.
Moderator
Draw a tree diagram, and different probability of each event occuring?

#### seventhroot

##### gg no re
not sure if square just seenzoned this or hasn't seen it
I seenzoned the title. From what I've seen in my 'experience' those threads just have some easy question or OP cbf doing it or something like that. (and I was right in this case)

Probably seenzoned it and said this topic got me in the HSC.

Thus, saying gg no re.
yeah I don't really like probability tbh. Too much reading

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Draw a tree and sum the probability or use the matlab package

imo very tedious question. If you decide to do the tree you're going to have 3^3 branches which is a lot. Are you using MIF or something?

#### braintic

##### Well-Known Member
An urn contains 10 blue balls, 5 green balls and 5 red balls. if 3 balls are drawn randomly without replacement, given that the first two balls are not red, what is the probability that the third ball is green?
No. of equally likely outcomes where first two balls are not red:
15 times 14 times 18 = 3780

No. of equally likely outcomes where first two balls are not red AND 3rd ball is green:
BBG: 10 times 9 times 5 = 450
BGG: 10 times 5 times 4 = 200
GBG: 5 times 10 times 4 = 200
GGG: 5 times 4 times 3 = 60
Total: 910

Probability = 910/3780 = 13/54

OR - as you seem to be a university student - use Bayes' theorem.

#### Queenroot

##### I complete the Squar3
I seenzoned the title. From what I've seen in my 'experience' those threads just have some easy question or OP cbf doing it or something like that. (and I was right in this case)
tbh even I could do this question

#### seventhroot

##### gg no re
tbh even I could do this question
it doesn't really matter? you don't have to do this "type" of maths again

#### Queenroot

##### I complete the Squar3
it doesn't really matter? you don't have to do this "type" of maths again
YAY

#### ConorDevaney

##### Member
P = (1 - red ball) x (5/19 + 4/19)