Beginning October 1, 1978, it shall also be the duty of the cities and fire districts of this state employing paid firefighters who are establishing the right to benefits under the provisions of chapter 14, title 72, Idaho Code, and of the boards and officers having authority therein, to cause to be remitted to the public employee retirement system board, as an incident to and part of the current expenses of such cities and fire districts, a sum equivalent to the total contribution rate and tax percentage paid into the Idaho public employee retirement system and the social security act on other public employees plus one percent (1%) thereafter of the average paid firefighter’s salary or wage in the state of Idaho, or the salary or wage of each individual firefighter, to be computed according to the classification of each firefighter under Option I or Option II as defined under section 72-1434, Idaho Code, for each paid firefighter employed by said cities or fire districts. The average paid salary or wage or individual firefighter’s salary or wage shall be measured and determined by the actual salary or wage earned during the twelve (12) month period beginning July 1 and ending June 30 immediately preceding September 1. Sums shall be remitted no later than five (5) days after each pay date as provided for remittances for individual firefighters as set forth in section 72-1431, Idaho Code. When a city or fire district is annexed by another city or fire district, the requirement of an annexed city or fire district to pay pursuant to this section shall transfer to the annexing city or fire district. The annexing city or fire district shall have the duty to cause to be remitted to the public employee retirement system board, as an incident to and part of the current expenses of such cities and fire districts, an amount as determined by the provisions of this section.