Probability

Oct 2019
1
0
Hyderabad
Find probability of choosing 2 black balls from 4 identical black balls and 7 identical red balls.
 
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mathman

Forum Staff
May 2007
6,895
760
$\frac{4}{11}\frac{3}{10}$.
 
Mar 2015
182
68
Universe 2.71828i3.14159
Find the probability of choosing 2 black balls from 4 identical black balls and 7 identical red balls.
Are you choosing balls one after another or at the same time? :rolleyes::rolleyes::p
 
Dec 2019
1
0
Utah
I'm confused. I have questions in my book about numeric probability like this: "A painter numbers copies of his paintings with a 4 digit number and makes enough copies to use all the possible 4-digit numbers. If you buy a painting, what is the probability of getting painting #5142?" In the answer key, it is 1/10,000 (10x10x10x10). I totally don't get this because if you have 4 digits to work with, you can only go up to 9. 10 is a 2-digit number. please explain how they are getting this answer.
 

romsek

Math Team
Sep 2015
2,776
1,552
USA
I'm confused. I have questions in my book about numeric probability like this: "A painter numbers copies of his paintings with a 4 digit number and makes enough copies to use all the possible 4-digit numbers. If you buy a painting, what is the probability of getting painting #5142?" In the answer key, it is 1/10,000 (10x10x10x10). I totally don't get this because if you have 4 digits to work with, you can only go up to 9. 10 is a 2-digit number. please explain how they are getting this answer.
0 is a digit. The digits 0-9 make up 10 possible digits.

Another thing different about this problem from the mailbox one is that order matters in mailbox numbers. 5142 is different from 2415.
If you had all the black balls labeled 1-4, then choosing ball 1 then ball 2 is equivalent to choosing ball 2 then ball 1. Both are still choosing 2 black balls.