An urn contains 3 white, 4 red and 5 black balls. Two balls re drawn one by one without replacement. What is the probability that at least one ball is black?

Explanation

A= in first draw A white or red ball comes up


B= in second draw A white or red ball comes up



`P(A)=frac{7}{12} `


`P(B / A)=frac{6}{11} `


`therefore P(A cap B)=P(A) times P(B / A) `


`=frac{7}{12} times frac{6}{11} `


`=frac{7}{22} `


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