What is the average year with an award of platinum?
CREATE TABLE "recording_industry_association_of_americ" (
  "year" real,
  "date" text,
  "title" text,
  "format_s" text,
  "award_description_s" text,
  "result_s" text
);
SELECT AVG("year") FROM "recording_industry_association_of_americ" WHERE "award_description_s"='platinum';