Traffic police monitor the speed of vehicles as they travel over a new bridge. The average speed for a sample of 41 vehicles was 81.35 km/h, with the sample standard deviation being 4.52 km/h. We will assume that the speeds are Normally distributed, and the police are interested in the mean speed. Part a) Since the variance of the underlying Normal distribution is not known, inference here would involve the t distribution. How many degrees of freedom would the relevant t distribution have