The annuity of an employee or Member who has had at least 5 years of service as a congressional employee, Member, or any combination thereof totaling 5 years is —

(a) One and seven-tenths percent of average pay multiplied by the total number of years of service as a Member and/or congressional employee not exceeding 20 years: plus

(b) One percent of average pay multiplied by the years of service other than that of a Member and/or congressional employee.