Answer
$lower~critical~value\lt r\lt upper~critical~value$: null hypothesis is not rejected.
There is not enough evidence to conclude that the arrival status is not random.
Work Step by Step
$H_0:~The~sequence~is~random$ versus $H_1:~The~sequence~is~not~random$
$n=15$, $n_l=5$, $n_o=10$ and $r=6$
Small sample case:
$lower~critical~value=3$
$upper~critical~value=12$
(According to table X, for $n_1=5$, $n_2=10$)
Test statistic: $r=6$
Since $lower~critical~value\lt r\lt upper~critical~value$, we do not reject the null hypothesis.
