Final answer:
A new account between accounts numbered 510 and 520 could be assigned number 515 if it is not used and complies with the organization's chart of accounts system.
Step-by-step explanation:
When adding a new expense account between accounts numbered 510 and 520, you could assign the new account number 515, assuming that this number is not already in use and follows the company's chart of accounts numbering system.
Typically, account numbers are assigned to organize financial information and make it easier to locate accounts within the general ledger. While 515 logically falls between 510 and 520, the actual number assigned would depend on the organization's unique accounting practices and whether they reserve certain numbers for specific types of transactions or future use.