The posted speed limit along Elm Street is 25 mph. Homeowners living on Elm Street claim that the average speed of automobiles traveling on Elm Street is greater than the posted limit of 25 mph and plan to
investigate their claim by measuring the speed of 20 randomly selected cars. If the data support the claim,
the residents plan to ask the city council for more police patrol along Elm Street. Here are the speeds (in
mph) of the 20 cars:
26.2 22.9 28.8 29.0 24.8
24.7 24.0 24.4 25.2 31.1
25.6 21.6 27.4 25.6 26.4
23.7 28.2 24.1 26.7 26.8

(a) Is there convincing evidence at the a = 0.05 significance level that the true mean speed of all cars
traveling on Elm Street is more than 25 mph?
(b) Referring to your conclusion, what type of error may have been made? Describe the error in the
context of this study.