What is the of Fleet size with a IATA of pr, and a Commenced operations smaller than 1941?
CREATE TABLE "commercial_airlines" (
  "airline" text,
  "icao" text,
  "iata" text,
  "callsign" text,
  "commenced_operations" real,
  "fleet_size" real,
  "headquarters" text
);
SELECT SUM("fleet_size") FROM "commercial_airlines" WHERE "iata"='pr' AND "commenced_operations"<1941;