What's wrong with my logic on this combinatorial/probability question?

  • Thread starter jdinatale
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  • #1
jdinatale
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Homework Statement



prob.png


The Attempt at a Solution



If at least 3 balls must be blue, then the possible ways of selecting at least 3 blues are:

r, b, b, b
b, r, b, b
b, b, r, b
b, b, b, r
b, b, b, b

where r = red ball, and b = blue ball

So there are 5 ways to draw 3+ blue.
 

Answers and Replies

  • #2
Dick
Science Advisor
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The question didn't really spell out what 'different' means. I think they want you to treat the balls as distinguishable and probably are not concerned with the order.
 
  • #3
bballninja
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The problem with your logic is that there are 9 different blue balls and you don't account for all of them being possibly drawn.

My hint for you is to consider combinations, i.e. how many ways are there to choose 3 blue balls out of 9?
 

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