Answer:
Explanation:
Let X be the length of time between billing dates and checks received and deposited for non-delinquent accounts of an oil company’s monthly credit card customers
X has mean 13.5 and sigma 3 days
Sample size = n =36, x bar = 13

(One tailed test at 0.025 sign. level)
Mean diff = -0.5
Since sigma is known z test can be used
Std error =

Z statistic = -0.5/SE= -0.1
p value = 0.920
p>0.025 hence accept null hypothesis.
My conclusion is there is no statistical evidence that lock box reduced the mean time.