Probability Homework.. help!

falconfighter

Member
Oct 10, 2004
46
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I just can't seem to get how to do this problem...
a bag has 500 marbles in it. 495 are red, 5 are green. What is the prob. of drawing at least 1 green if you draw 50 marbles?
If you draw 75 marbles?
I have no idea where to go at this. Help!
 

vtqanh

Diamond Member
Jan 4, 2001
3,100
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76
If I remember how to do this problem correctly:

Probability of red is 495/500, probability of green is 5/500
P (Drawing at least 1 green) = 1 - P (Drawing no green) = 1 - P (Drawing all 50 red)
= 1 - (495/500)^50 * (5/500)^ 0 ~ 40%

The 75 case should be similar
 

Modeps

Lifer
Oct 24, 2000
17,255
44
91
Originally posted by: vtqanh
Probability of red is 495/500, probability of green is 5/500
P (Drawing at least 1 green) = 1 - P (Drawing no green) = 1 - P (Drawing all 50 red)
= 1 - (495/500)^50 * (5/500)^ 0 ~ 40%

The 75 case should be similar

He's never gonna learn if you do it for him!
 

Lonyo

Lifer
Aug 10, 2002
21,939
6
81
Use your calculator.
There's a function for this.
pdfbin or similar (probability something binomial blah)
Or you might want cdfbin or similar (cumulative blah binomial somesuch).
And then put in 3 numbers or 2 numbers or something.
And make the calculator work it out for you.

<-- is glad he got 95% on his Stats in January so he could forget it ALL.
 

TheChort

Diamond Member
May 20, 2003
4,212
0
76
Originally posted by: vtqanh
If I remember how to do this problem correctly:

Probability of red is 495/500, probability of green is 5/500
P (Drawing at least 1 green) = 1 - P (Drawing no green) = 1 - P (Drawing all 50 red)
= 1 - (495/500)^50 * (5/500)^ 0 ~ 40%

The 75 case should be similar

you forgot the fact that the probability changes for each marble he draws
 

Atomicus

Banned
May 20, 2004
5,192
0
0
(495 choose 49) times (5 choose 1)

that divided by 500 choose 50

for the 75 draw case, change numbers accordingly
 

oog

Golden Member
Feb 14, 2002
1,721
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0
Originally posted by: vtqanh
If I remember how to do this problem correctly:

Probability of red is 495/500, probability of green is 5/500
P (Drawing at least 1 green) = 1 - P (Drawing no green) = 1 - P (Drawing all 50 red)
= 1 - (495/500)^50 * (5/500)^ 0 ~ 40%

The 75 case should be similar

that's not right, is it? if you're drawing 50 marbles, the probably of drawing each successive red changes a bit. it may start at 495/500, but the very next one will be 494/499. this shifting probability with each successive marble throws off your (495/500)^50 term. i'll leave it to the OP to determine what to do next.