Need help Tuning this Query

Oracle Version 8.1.7 and optimizer_mode=Rule

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insert into Temp_Credit (location,full_name,status,term_name,loan_name,amount,

     person_id,score,scorerange,arbalance,actual_balance,

     cost_id,roster_number,STUDENT_INSTANCE_ID,new_start_ind,

     Packaged_Date,PrePackaged_Date)

    SELECT LOCATION, full_name, status, term_name, loan_name, amount,

           tc.person_id, null,null,AR_Balance(tc.person_id,a.zone_name,P_MonthEnd),

           Actual_Balance(tc.person_id,a.zone_name),

            cost_id,roster_number,a.student_instance_id,

            New_Start(tc.student_instance_id,'SSS',P_NewStartDate),

            decode(report_req_date(tc.student_instance_id,'SSS',25),'N',null,