Please answer the Exercise 3.15, 3.18 and 3.21 with your detailed explanations, it's much appreciated!

Note: For 3.18, fill in the blank by choosing one of credit, Sales invoice and Supplier(purchase) invoice

3.15 What is the double entry to record credit purchases in the general ledger? Debit purchases, Credit bank Debit trade payables, Credit purchases Debit bank, Credit purchases Debit purchases, Credit trade payables When businesses trade, there are many different documents used to record the different stages of each transaction. Certain documents are key sources of information in an accounting system. A new accounts assistant is being trained on the payables ledger. From the perspective of the supplier, identify the names of the business documents described below. 21 From time to time, customers return goods to suppliers. They may then send a formal request for the supplier to issue a credit note. Which business document is used by customers to send a formal request for a credit note? Credit note Debit note Remittance advice Quotation