[Answer] A random variable X has a mean of 120 and a standard deviation of 15. A random variable Y has a mean of 100 and a standard deviation of 9. If X and Y are independent approximately what is the standard deviation of X - Y ?

Answer: 17.5

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A random variable X has a mean of 120 and a standard deviation of 15. A random variable Y has a mean of 100 and a standard deviation of 9. If X and Y are independent approximately what is the standard deviation of X - Y ? Definitions Generation and parameters. Let be a standard normal variable and let and > be two real numbers. Then the distribution of the random variable = + is called the log-normal distribution with parameters and .These are the expected value (or mean ) and standard deviation of the variable s natural logarithm not the expectation and standard deviation of itself. Suppose one wishes to calculate Pr( X ≤ 8) for a binomial random variable X . If Y has a distribution given by the normal approximation then Pr( X ≤ 8) is approximated by Pr( Y ≤ 8.5). The addition of 0.5 is the continuity correction; the uncorrected normal approximation gives considerably less accurate results. Central limit theorem - Wikipedia Binomial distribution - Wikipedia Sumof normally distributed random variables - Wikipedia Ratio distribution - Wikipedia Given two (usually independent ) random variables X and Y the distribution of the random variable Z that is formed as the ratio Z = X / Y is a ratio distribution. An example is the Cauchy distribution (also called the normal ratio distribution) [citation needed] which comes about as the ratio of two normally distributed variables with zero mean . Definitions Probability mass function. A discrete random variable X is said to have a Poisson distribution with parameter > if it has a probability mass function given by:: 60 (;) = (=) = −! where k is the number of occurrences (= ; e is Euler's number (=! is the factorial function.; The positive real number λ is equal to the expected value of X and also to its variance The mean or expected value of an exponentially distributed random variable X with rate parameter λ is given by ⁡ [] =. In light of the examples given below this makes se...

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