What is the average total donations received by Fresno County colleges?
Fresno County colleges refers to school_county = 'Fresno'; average refers to DIVIDE(sum(donation_optional_support,donation_to_project),sum(donation_total))
SELECT SUM(T2.donation_optional_support + T2.donation_to_project) / COUNT(donationid) FROM projects AS T1 INNER JOIN donations AS T2 ON T1.projectid = T2.projectid WHERE T1.school_county = 'Fresno'