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Problem of the Day #389: Blue or Red April 11, 2012

Posted by Saketh in : potd , trackback

Billy places $10$ marbles in a jar, $5$ blue and $5$ red. When he isn’t looking, Arjun comes along and takes one of the marbles.

Billy comes back and draws some number of marbles from the jar, with replacement. He observes that the number of times he drew a blue marble is $5$ greater than the number of times he drew a red marble.

What is the probability that the marble Arjun took was blue?


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