A county now paves its highways using Asphalt A, but is considering switching to the more pricy Asphalt B. (which is reported to be more durable). On 5 different two-lane roadways, the two types of asphalt are evaluated using 200m test strips. A randomly selected lane on each test strip was paved with Asphalt A, whereas the lane in the other direction was paved with Asphalt B. It has been shown that using Asphalt B is more cost effective if the period between resurfacing (i.e. longevity) is at least 2.5 years longer on average than using Asphalt A. Table 1 summarises the information. Assume that the longevity of each asphalt type is normally distributed.

Table 1: Longevity (in years) of 200m asphalt test-strips on 5 rural two-lane highways Location 2 3 4 5 Mean Std. Dev Asphalt A 11.2 9.0 10.7 11.4 8.5 10.16 1.32 Asphalt B 18.8 11.7 19.9 19.0 13.4 16.56 3.73 D = XB - XA 7.6 2.7 9.2 7.6 4.9 6.40 2.58