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# Probabilities : Marginals

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A bag contains r balls, of which 2 are red and r - 2 are black. We draw a ball at random, we write down its color, and we put it to one side (not back in the bag). We repeat the procedure r times.
Let X be the number of draws needed to obtain a red ball, and let Y be the number of draws needed to obtain a second red ball.
a. Find the probability law of the vector (X,Y) , and find its marginals.
b. Evaluate Cov(X,Y) .

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Probailities are calculated using appropriate tables and calculations. The solution is detailed.

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A bag contains balls, of which 2 are red and are black. We draw a ball at random, we write down its colour, and we put it to one side (not back in the bag). We repeat the procedure r times.
Let X be the number of draws needed to obtain a red ...

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###### Education
• BSc , Wuhan Univ. China
• MA, Shandong Univ.
###### Recent Feedback
• "Your solution, looks excellent. I recognize things from previous chapters. I have seen the standard deviation formula you used to get 5.154. I do understand the Central Limit Theorem needs the sample size (n) to be greater than 30, we have 100. I do understand the sample mean(s) of the population will follow a normal distribution, and that CLT states the sample mean of population is the population (mean), we have 143.74. But when and WHY do we use the standard deviation formula where you got 5.154. WHEN & Why use standard deviation of the sample mean. I don't understand, why don't we simply use the "100" I understand that standard deviation is the square root of variance. I do understand that the variance is the square of the differences of each sample data value minus the mean. But somehow, why not use 100, why use standard deviation of sample mean? Please help explain."
• "excellent work"
• "Thank you so much for all of your help!!! I will be posting another assignment. Please let me know (once posted), if the credits I'm offering is enough or you ! Thanks again!"
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