Probability

goofyfootsp

1. The problem statement, all variables and given/known data

A bag contains 40 blue marbles, 40 red marbles, 20 green marbles. 60 of the marbles are big. Three marbles are selected without replacement.

What is the probability of randomly selecting three big blue marbles?

2. Relevant equations

p(x,y/theta=p(x)p(y/x)=p(y)p(x/y)

3. The attempt at a solution

(40/60)(39/59)(38/58)/(40/100)(39/99)(38/98)

lzkelley

first being big blue: p1 = (40/60)(60/100)
second: p2 = (39/60)(59/99)
third: p3 = ...

all 3 = p1*p2*p3
... right?

exk

wouldn't p2 = (39/59)(59/99) since there is 1 less big marble left?

lzkelley

def (thats what i intended), sorry.
p2 = (39/59)(59/99)

goofyfootsp

after multiplying p1*p2*p3 do I need to divide the sum by another number?

exk

Why would you think that you need to divide by another number?

goofyfootsp

Because the answer when p1*p2*p3=.061. yet my worksheet has choices of a. .0125, b. .0240, c. .2323, d. .2400

exk

well on second thought you are write, it can't be the above.

B=blue, b=big.

P(B)*P(b) is the probability of the marble being big and blue and it initially = 40/100*60/100 since being big and being blue are independent of each other.

So I get 40/100*60/100*39/99*59/99*38/98*58/98 for all 3 to be B&b, since they are drawn in succession and without replacement. That = 0.0129, which is close to A?

goofyfootsp

Thanks I got it and I really appreciate your help!

exk

so was A the answer? I am just curious.

goofyfootsp

yes thanks!