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Probability

Question
CBSEENMA12033814

Three cards are drawn successively with replacement from a well-shuffled deck of 52 cards. A random variable X denotes the number of spades in three cards. Determine the probability distribution of X.

Solution
Here X takes the values 0, 1, 2, 3.
Let p denote the probability of getting spade.
Now,    straight p space equals 13 over 52 space equals 1 fourth comma space space space space straight q space equals space 1 minus space 1 fourth space equals space 3 over 4
P(X = 0) = straight q space cross times space straight q space cross times space straight q space equals space 3 over 4 cross times 3 over 4 cross times 3 over 4 space equals space 27 over 64
P(X = 1) = straight p space cross times space straight q space cross times space straight q space plus space straight q space cross times space straight p space cross times straight q space plus space straight q space cross times straight q space cross times straight p
               equals space 1 fourth cross times 3 over 4 cross times 3 over 4 plus 3 over 4 cross times 1 fourth cross times 3 over 4 plus 3 over 4 cross times 3 over 4 cross times 1 fourth space equals space 9 over 64 plus 9 over 64 plus 9 over 64 equals 27 over 64
P(X = 2) = straight p space cross times space straight p space cross times space straight q space space plus space straight p space cross times space straight q space cross times space straight p space plus space straight q space cross times space straight p space cross times space straight p
               equals space 1 fourth cross times 1 fourth cross times 3 over 4 plus 1 fourth cross times 3 over 4 cross times 1 fourth plus 3 over 4 cross times 1 fourth cross times 1 fourth space equals space 9 over 64
P(X = 3) = straight p space cross times space straight p space cross times space straight p space equals 1 fourth cross times 1 fourth cross times 1 fourth space equals space 1 over 64
∴ probability distribution is
     

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