[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. Tue Feb 20 2007 · In statistics the 68–95–99.7 rule also known as the empirical rule is a shorthand used to remember the percentage of values that lie within a band around the mean in a normal distribution with a width of two four and six standard deviations respectively; more precisely 68.27% 95.45% and 99.73% of the values lie within one two and three standard deviations of the mean respectively. Central limit theorem - Wikipedia Variance - Wikipedia Continuous uniform distribution - 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 . 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 sense: if you receive phone calls at an average rate of 2 per hour then you can expect to wait half an hour for every call.. The variance of X is given by ⁡ [] = so the standard deviation is equal to the mean . Definitions Probability mass function. A discrete random variable X is said to have a Poisson distribution with parameter > if it has a p...

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