A sack of balls has 3 red and 5 blue.
If you select 2 balls, what is the probability that none of them are red?

Ideas: I know how to find the probability with 1 selection, but I'm not sure about 2, it sounds like the 2nd ball is a conditional probability, and do we add the 2 probabilities together or multiply them? :o

Question

A sack of balls has 3 red and 5 blue.
If you select 2 balls, what is the probability that none of them are red?

Ideas: I know how to find the probability with 1 selection, but I'm not sure about 2, it sounds like the 2nd ball is a conditional probability, and do we add the 2 probabilities together or multiply them? :o

mantar0078 · Answer

5/8?

mantar0078 · Answer

If you multiplies the probabilities by two then you would have to simplify them back down to what they originally were...

mhchen · Answer

Well actually the answer key is 5/14. It didn't say how it got it though.

directrix · Answer

There are 8 balls in the urn. The number of balls that are NOT red are the 5 blue balls.

On the first draw, the probability of getting a blue ball is 5/8.

Once you draw a blue ball, there are 4 left in the urn. There are 7 mixed color balls in the urn.

On the second draw, the probability of getting another blue ball is 4/7.

Putting the two draws together,  the probability that none of them are red is this:

5/8 *  4/7 = what?

directrix · Answer

@mhchen

triciaal · Answer

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