The amount of the monthly ordinary service pension payable to a retired member shall be the sum of 2.5% of final average compensation multiplied by years of service up to 20 years inclusive, plus 3.5% of final average compensation multiplied by years of service above 20 years.

64 Del. Laws, c. 445, § 1; 70 Del. Laws, c. 186, § 1; 75 Del. Laws, c. 133, § 1