Consider the experiment of rolling a pair of fair dice until a sum of 7 is obtained. Let X denote the number of trials needed to obtain a sum of 7.
a) Notice that X is discrete. What is the distribution of X?
b)What is the theoretical average value (mu) of X?
c)What is the theoretical variance of X?
d) Consider the random variable Y, the average number of trials needed to roll a sum of 7 based on 25 trials. What is E[Y] and Var (Y)?

Respuesta :

a) The distribution of X is a discrete geometric distribution with pmf p(x) = (5/36)(1/6)^(x-1).b) The theoretical mean (mu) of X is 6.c) The theoretical variance of X is 35.d) The expected value of Y is 6 and the variance of Y is 3.875.

The distribution of X is a discrete geometric distribution, which means that the probability of X taking on any particular value is given by the formula p(x) = (5/36)(1/6)^(x-1). The theoretical mean (mu) of X is 6, which means that on average it should take 6 trials to obtain a sum of 7. The theoretical variance of X is 35, which means that there is a considerable amount of variability in the number of trials needed to obtain a sum of 7. If we consider the random variable Y, the average number of trials needed to roll a sum of 7 based on 25 trials, then the expected value of Y is 6 and the variance of Y is 3.875. This indicates that the average number of trials needed to obtain a sum of 7 is 6 and that the variability decreases as more trials are taken.

a) p(x) = (5/36)(1/6)^(x-1)

b) theoretical mean (mu) of X mu = 6

c) Var(X) = (5/36)*∑x=1∞(1/6)^(x-1)^2 - mu^2

       = (5/36)*(1-1/36^2) - 6^2

       = 35

d) The expected value of Y , E(Y) = 6

Var(Y) = Var(X/6)

       = Var(X)/6^2

       = 35/6^2

       = 3.875

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