2 Unit probablity (1 Viewer)

[Affinity]

this is from a 2 unit exam, see how many of you can do it

there are 2 red, 1black and 1 white marbles in a bag. two marbles are drawn one after the other, and kept hidden in a hand.

a) find the probability that that 2 red marbles are drawn

b) find the probabilty that the second marble is red.

c) one of the drawn marbles slips from the person's hand and it was red, what is the probability that the other one is also red.

 

[underthesun]

Well, a crude attempt, but:

a) 2/4 * 1/3 = 1/6

b) 2 x 1/4 x 2/3 + 1/6 = 1/3 + 1/6 = 1/2

c) = 1/6 / 1/2 = 1/3

 

[Affinity]

try again, one of your answers is wrong

 

[ezzy85]

is part b 1/2 X 2/3 = 1/3?

 

[Affinity]

part b is 1/2

part c is wrong

 

[Affinity]

part c meant that after drawing the 2, 1 falls and it was seen to be red.

 

[crazylilmonkee]

wouldnt it just be 1/3 since the one dropped was red.. for the second one to be red it would just be 1 red from the remaining 3 balls

 

[...]

is c) 1/2??

edit: wait..

hmm..
p(red_1) = 1/4
p(red_2) = 1/4 too

err...u times it...so its 1/8..

woah
my second answer look way off...

 

[Affinity]

no.. the 2 balls are first drawn and then one fell down

 

[crazylilmonkee]

i've done this before!!
i should know it!!
*rummages thru my notes*

 

[Affinity]

c'mon guys/gals

it's not 1/2 not 1/3 not 1/8

 

[...]

1/6!!!

cos first chance of picking a red is 0.5
second is 1/3...

 

[Affinity]

not 1/6 either

 

[crazylilmonkee]

its 2/3 !!!
isnt it?
i couldnt find my notes!!

 

[Affinity]

2/3? thats getting colder

 

[Archman]

1/5?

 

[crazylilmonkee]

cos it has to be 1/3 but was it the first ball or second ball? so u multiply by 2!!

 

[...]

Originally posted by Affinity
2/3? thats getting colder

any hints

 

[crazylilmonkee]

gayyyyyyyyy
wats the damn answer

 

[Affinity]

archman got it !