After allowing a customer to buy on credit ,what internal control methods would you apply in order to maintain control over your debtors

When you allow a customer to buy on credit, this means that the customer has passed stringent credit worthy standards. The company have done background and credit checks and other checks needed to verify the ability and the credit worthiness of the customer.

After the customer's purchase on credit, you can do the following to maintain control over your debtors:
1) Each debtor must have an individual updated debtors account.
2) give discounts to debtors who pay early
3) sent out periodic reminders on credit due.
4) charge interest on overdue accounts
5) limit bad debts
6) No sales to overdue debtors
7) close the accounts of long overdue debtors

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It took me a while to figure out this complex expression but i believe the answer you’re looking for is

but i am not sure because i am not at that level of math yet.