Select Invoice_number, invoice_due_date, payment_date and the number of days late for those invoices that were paid after the due date. (give this a meaningful name using AS) Then using that WHERE...

Select Invoice_number, invoice_due_date, payment_date and the number of days late for those invoices that were paid after the due date. (give this a meaningful name using AS) Then using that WHERE clause, when clients paid late, find the average number of days they were late.