SDCL § 54-3A-8: Rule for computing refund of charges on prepayment.

As used in this chapter, unless the context otherwise requires, "Rule of 78" means a refund of charges which shall be at least as great a proportion of the total charges, as the sum of the remaining monthly balances of principal and interest combined scheduled to follow the date of prepayment bears to the sum of all the monthly balances of principal and interest combined originally scheduled by the loan agreement.