please help A city planner must contract a company to widen the highway if the a

Question

please help

A city planner must contract a company to widen the highway if the average rush hour speed dips below 40 mph. He takes a smile of 81 cars over a two-week period and finds that the average speed was 37 mph with a standard deviation of 15 mph. Is the mean rush hour speed of all cars significantly less than 40 mph? Determine the critical value (s) if alpha = 10 a. z_alpha/2 = plusminus 1.645 b. z_alpha = 1.28 c. z_alpha = -1.28 d. t_alpha/2 = plusminus 1.664 e. t_alpha = 1.292 f. t_alpha = -1.292

Explanation / Answer

The standard deviation of population is not known.

and instead we have taken the sample standard deviation.

so, we consider t distribution . instead of Normal

then, since our test is one sided, we take

talpha instead of alpha/2.

t0.05= 1.292

that's the right answer.

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