(1)    Installments allowed. Delinquent property taxes, special assessments, special charges and special taxes may be paid to the appropriate treasurer in partial payments of not less than $20, unless the treasurer agrees to accept a lower amount.

   (2)   Principal and interest.

      (a)    The treasurer shall determine that portion of a partial payment to be applied as principal by dividing the amount of the partial payment by a figure which is the sum of one plus a figure which is the product of the number of months of delinquency, as determined under s. 74.11, 74.12 or 74.87:

         1.    Times 0.01, if no penalty under s. 74.47 (2) applies; or

         2.    Times a decimal which reflects the applicable percentage, if a penalty under s. 74.47 (2) applies.

      (b)    The amount of the payment that is in excess of the amount of principal determined under par. (a) is the interest and penalty accrued from the date of the delinquency on the amount of the partial payment which is principal. After any partial payment is made, subsequent determinations of interest and penalty shall be computed only on the unpaid balance of the principal, from the date of the delinquency as determined under s. 74.11, 74.12 or 74.87.