Suppose that an auditor is auditing the financial transactions of ABC Company. There are 4000 sales invoices that require auditing. The auditor grouped the invoices into three groups as small transactions (group 1), medium transactions (group 2) and large transactions (group 3). Group 1 contains 1000 invoices, group 2 contains 1500 invoices and group 3 contains 1,500 invoices. The researcher wants to select 20% of invoices from each group.