not sure which on to choose

) A tyre manufacturer claims that his new steel-belted tire has a mean life expectancy of 40,000 miles. A consumer association decides to test these claims against the alternatives that the mean life expectancy is less than 40,000 miles. 100 tyres are selected at random and the sample mean is found to be 38,500 miles with the sample standard deviation of 2,000 miles.

Perform the hypothesis test at the 5% (or 0.05) level of significance.

because its a sample standard deviation i should use xbar- meu = a and then SD/ n square root= b

so is it 38500- 40000 = 1500 and the 2000 / sq of 100 = 10 = 2000/ 10 = 200 so

1500/ 200 = 7.5

so

h0 meu= 40000

h1 meu doesnt=40000

5%