Question 1161975
Ho: S=C mpg
Ha: they are not equal
alpha is 0.05 p{reject Ho|Ho true}
test is a t but df is more complicated, Could use 15+8-2=21. This is an issue, because others will say to use the smaller sample size in which case the df would be 7.
reject if |t|> critical value
With a calculator, the complicated df can be dealt with
t=(s-c)/ sp*(1/sqrt(n1)+1/sqrt(n2))
STAT TESTS 4 ENTER
with df=21 the critical value is 2.086
t=1.83 which is not greater than the critical value. Fail to reject based on these data. 
p-value is 0.10.
Here, the variances are not likely to be equal, which makes the df calculation more difficult. The calculator gave a more conservative estimate of about 8.5. With equal variances, the df would be 21.