Answer
$k\gt critical~value$: null hypothesis is not rejected.
There is not enough evidence to conclude that the median number of miles on 3-year-old Impalas is less than 40,428 miles.
Work Step by Step
$H_0:~M=40,428$ versus $H_1:~M\lt40,428$
$number~of~plus~signs=6$
$number~of~minus~signs=12$
$n=6+12=18$ (small sample)
$Critical~value=5$
(According to table XI, for $n=18$ and $α=0.05$ (one tail))
Left-tailed test:
$k=number~of~plus~signs=6$
Since $k\gt critical~value$, we do not reject the null hypothesis.
