What is average quantity, when DRG number is 99 011?
CREATE TABLE "narrow_gauge_locomotives" (
  "class" text,
  "railway_number_s" text,
  "drg_number_s" text,
  "quantity" real,
  "year_s_of_manufacture" text,
  "axle_arrangement_uic_bauart" text
);
SELECT AVG("quantity") FROM "narrow_gauge_locomotives" WHERE "drg_number_s"='99 011';