Question 1120540
Ho: mean is >=35
Ha: mean is not
alpha=0.05
test stat is a t 0.95, df=39
critical value t < -1.69. If the test statistic is less than -1.69, it will be considered to be outside the possibility of a chance random effect.  One way test
test statistic t=(33-35)/3.8/sqrt (40)  (mbar-mean)/s/sqrt(n)
t=-2*sqrt(40)/3.8=-3.33. This is the number of sd s in the t-distribution for data with a mean of 35 and s of 3.8 with df=39.
p value=0.001.  If the null hypothesis were true, the probability of getting a result of this magnitude or less is one in a thousand.
Reject Ho and conclude that the mean is less than 35 mpg.  

There are many such reasons for this including the dealer is not telling the truth; that is not a statistical conclusion given what is given.