(a) In order to compute payment by surrendering bonds, the interest rate on the bonds shall be applied on an annual basis to each unpaid installment of taxes for the number of years the installment must run before being due. The total of the items computed shall be deducted from the face amount of the unpaid installments of taxes.
(b) In order to compute payment by surrendering bonds, the interest rate on the bonds shall be applied to each unpaid installment of taxes for the number of years the installment has been past due and 10 percent of the face amount of each installment of taxes that is past due shall be added as a penalty. The total of the items computed shall be added to the face amount of each unpaid installment of taxes.