The manager of a fleet of automobiles is testing two brands of radial tires and assigns one tire of each brand at random to the
two rear wheels of eight cars and runs the cars until the tires wear out. The data (in kilometers) follow. Find a 99% confidence interval on the difference in the mean life. Car Brand 1 Brand 2
1 37734 35202
2 45299 41635
3 36240 35500
4 32100 31950
5 37210 38015
6 48360 47800
7 38200 37810
8 33500 33215
(a) Calculate d=
(b) Calculate sD =
(c) Calculate a 99% two-sided confidence interval on the difference in mean life.
Explanation: the total cost if the rental is a linear function of the number of days of the rental (x). Each day costs $25, so that is the slope of the line. Since there is also a one-time fee, independent of the number of days, this amount $50 represents a vertical shift of the line on the graph, by 50. y=25x+50 is an algebraic expression of that.