A stationary shop finds that 20% of the customers entering the shop between 9 am to 3 pm make a

purchase. One morning, 100 people who can be regarded as a random sample of all customers, enter the

shop. Out of them, 15 people made a purchase.

a) What is the standard error of the sample proportion?

b) What is the sample under consideration?

c) Test the null against the alternative that the population proportion of people making a purchase is less

than 20%.