Question 1188604
This is testing means of a variable (mileage) against a specific mean. The sd can be calculated and if assumed to be an unbiased estimator of the population's, than a 1-sample t-test would be appropriate.
Ho: mileage= 30000
Ha: mileage ne 30000
alpha; 0.10 p{reject Ho|ho true}=0.10
test is a t-test (0.90, df=20)
critical value |t|>1.725
calculation the mean is 
33950
s=11866
the output on a calculator t=1.53 p-value =0.14
also
t=(33950-30000)/11866/sqrt(21)
=3950*sqrt(21)/11866
=1.53
fail to reject Ho, insufficient evidence to deny the claim.