What is the average decile of Ruapehu college, which has a state authority?
CREATE TABLE "ruapehu_district" (
  "name" text,
  "years" text,
  "area" text,
  "authority" text,
  "decile" real,
  "roll" real
);
SELECT AVG("decile") FROM "ruapehu_district" WHERE "authority"='state' AND "name"='ruapehu college';