Answer
We are 95% confident that the mean number of shares of Harley Davidson traded per day in 2010 is between 1.978 million and 2.412 million.
Work Step by Step
Here we have n = 40, df = n-1 = 39, x̅ = 2.195 and s = 0.815
Confidence = 90%
α = 1 - 0.90 = 0.10, α/2 = 0.05, $t_{α/2} = 1.685$
$E = 1.685 \times \frac{0.815}{\sqrt 39} = 0.217 $
Lower Bound = x̅ - E = 2.195 - 0.217 = 1.978
Upper Bound = x̅ + E = 2.195 + 0.217 = 2.412
