What is the total sum of the donations paid with an optional support in projects that reach more than 300 students?
with an optional support refers to donation_included_optional_support = 't'; reach more than 300 students refers to students_reached>300
SELECT SUM(T2.dollar_amount) FROM projects AS T1 INNER JOIN donations AS T2 ON T1.projectid = T2.projectid WHERE T1.students_reached > 300 AND t2.donation_included_optional_support = 't'