please explain in details

You need to select a sample of accounts receivables to test for existence and a sample to test for valuation given the following information:

About 70% of total sales are paid by cash or credit cards while 30% are on credit. The Apia head office deals with these accounts and the responsibility for accounts receivables collection. An alphabetical accounts receivables list is produced each month, showing the invoices making up each debtor, payment details and the outstanding balance.

Describe how you would select the samples to ensure the audit is performed in an efficient and effective manner.