If any taxpayer fails to pay the tax required by this chapter by the last day of the month in which the tax becomes due as herein provided then in addition to such tax a penalty of 10 percent of the amount of said tax together with interest at the rate of six percent per annum on the tax due, said interest to be computed from the date said tax became due until the same is paid. (Ord. 231 § 7, 1961)