



 
Hi Patrick, I am going to look at a different example. Suppose I have 5 coins in my pocket, one worth 25 cents, two worth 10 cents each and two worth 5 cents each. I select a coin at random and the random variable X is the value of the coin I select. The probability distribution is then
To find the expected value of a random variable you multiply each possible value of the variable by the probability that you obtain that value and then add the resulting numbers. Thus the expected value of X is
The standard deviation of a random variable is the square root of the variance and the variance is defined as the expected value of the random variable (X  E(X))^{2}. For my example E(X) = 11¢ and hence the random variable (X  E(X))^{2} and its probability distribution is given by
Thus
and
Now try your problem. Harley  


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