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Some manufacturers claim that non-hybrid sedan cars have a lower mean miles-per-
gallon (mpg) than hybrid ones. Suppose that consumers test 21 hybrid sedans and get
a mean of 31 mpg with a standard deviation of seven mpg. Thirty-one non-hybrid
sedans get a mean of 22 mpg with a standard deviation of four mpg. Suppose that
the population standard deviations are known to be six and three, respectively.
Conduct a hypothesis test to evaluate the manufacturers claim.