From the total amount of donation to projects, what is the percentage of the amount is for school projects located in the rural area?
located in the rural area refers to school_metro = 'rural'; percentage = divide(sum(donation_to_project), sum(donation_to_project where school_metro = 'rural'))*100%
SELECT CAST(SUM(CASE WHEN T2.school_metro = 'rural' THEN T1.donation_to_project ELSE 0 END) AS REAL) * 100 / SUM(donation_to_project) FROM donations AS T1 INNER JOIN projects AS T2 ON T1.projectid = T2.projectid