My Math Forum Probability/Combinations confusion

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 October 19th, 2016, 05:38 AM #1 Newbie   Joined: Oct 2016 From: United States Posts: 1 Thanks: 0 Probability/Combinations confusion Hi everyone, I'm having some difficulty understanding when to use combinations and when to just multiply specific probabilities together for a given outcome... So we have 10 marbles, 7 red, 3 blue. How do I find the probability of getting four red marbles in a row? I tried two methods and they both seem correct to me, but they give me different results and I'm not sure why. Probability of desired event = Number of desired events/ number of total events (7/10) * (6/9) * (5/8 ) * (4/7) = 840/5040 = .166 -or- (number of ways to choose 4 red marbles) = 1 (number of total ways to chose 4 marbles out of 10) = (10 choose 4) = (10*9*8*7)/(4*3*2*1) = 5040/24 = 210 1/210 = .0049 I'm not sure which is correct, and why the two methods give me different answers. Please Help! Last edited by skipjack; August 7th, 2017 at 08:49 PM.
 August 7th, 2017, 08:52 PM #2 Global Moderator   Joined: Dec 2006 Posts: 19,168 Thanks: 1640 The first is correct.

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