Chapter 9 - Section 9.2 - Consumer Reports - Consumer Reports Tests Tires - Page 454: c

We are 95% confident that mean braking distance is between 127.92 and 136.324 feet.

Here we have n = 9, df = n-1 = 8, x̅ = 132.122 and s = 5.467 Confidence = 95% α = 1 - 0.95 = 0.05, α/2 = 0.025, $t_{α/2} = 2.306$ $E = 2.306 \times \frac{5.467}{\sqrt 9} = 4.202 $ Lower Bound = x̅ - E = 132.122 - 4.202 = 127.92 Upper Bound = x̅ + E = 132.122 + 4.202 = 136.324