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probability that not all balls are orange?

Problem Solving — algebra and arithmetic (GMAT Focus Edition)
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by r_walid » Fri Nov 13, 2009 1:17 am
Say I have a basket with 5 blue and 6 orange balls, I am to pick 4 balls at random.

what is the probability of not picking all 4 balls as orange?

please explain with detail your reasoning.

Also, I am wondering, in this case, what is the general formula of this problem? i.e, the sum of different probabilities = 1 for every situation. In this case, what is the formula? is it P(all orange) + P(all blue) + P(orange/blue) = 1??? I'm not sure if this is correct, let me know what you think.

Thanks.
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by usfgrad » Fri Nov 13, 2009 1:36 am
You would figure out hte probabilty of picking all orange and then subtract that number from one - it is the fastest way
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by Kalvin » Wed Nov 18, 2009 5:19 pm
Probability of picking all 4 balls as orange = (6/11)(5/10)(4/9)(3/8) = 1/22 after cancelling

Probability of not picking all 4 balls as orange = 1 - (1/22) = 21/22
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by Leon1984 » Mon Nov 23, 2009 11:03 pm
Why not can just get the probability of getting 1 blue?
(5/11+5/10+5/9+5/8)
Leon
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