What is the average Grid for the honda cbr1000rr, with 18 laps and a time of +1:12.884?
CREATE TABLE "superbike_race_1_classification" (
  "rider" text,
  "bike" text,
  "laps" real,
  "time" text,
  "grid" real
);
SELECT AVG("grid") FROM "superbike_race_1_classification" WHERE "bike"='honda cbr1000rr' AND "laps"=18 AND "time"='+1:12.884';