let x be a random variable whose values are the number of dots that appear on the uppermost face when a fair die is rolled. the possible values of x are 1, 2, 3, 4, 5, and 6. the mean of x is 7/2 and the variance of x is 35/12. let y be the random variable whose value is the difference (first minus second) between the number of dots that appear on the uppermost face for the first and second rolls of a fair die that is rolled twice. what is the standard deviation of y ?