Qick Express is responding to criticisms from customers about late delivery of parcels by initiating new procedures to alleviate this problem. They have been assured that this change would result in a late rate of 10 percent. After 6 months of using the suggested new procedures, Qick Express decided to conduct an investigation into the late delivery claim by posting 21 parcels to various parts of the country. 5 parcels arrived late at their destinations.

b) Explain why a 95 percent confidence interval can be used in hypothesis testing at 5 percent significance level.