Name the total number of founded for public and 780 enrollment
CREATE TABLE "past_members" (
  "institution" text,
  "nickname" text,
  "location" text,
  "founded" real,
  "type" text,
  "enrollment" text,
  "current_conference" text
);
SELECT COUNT("founded") FROM "past_members" WHERE "type"='Public' AND "enrollment"='780';