A car manufacturer claims that the highway mileage for their car is $18$kmpl$.$ An analyst wants to test this claim at a siginificance level of $0\mathrm{.}05.$ The $30$ randomly selected cars recorded mean highway mileage of $15$ kmpl$.$ The population standard deviation is $2$ kmpl$.$ Conduct the hypothesis test and state the assumptions, hypotheses, test statistic, and decision for this test.