701—10.103 (422) Interest on unpaid tax. Tax not paid within the time prescribed by law, including the period during an extension of time, draws interest at the rate of three-fourths of one percent per month for each month, or fraction of a month, that the tax liability remains unpaid until December 31, 1981. See rule 701—10.2(421) for the statutory interest rate commencing on or after January 1, 1982. Payments made are first credited to penalty and interest due and then to the tax liability. See Ashland Oil Co.	v.	Iowa Department of Revenue and Finance, 452 N.W.2d 162 (Iowa 1990).