What is the max speed of the unit with an 8+4 quantity built before 1971?
CREATE TABLE "diesel_multiple_units" (
  "type" text,
  "numbers" text,
  "year_built" real,
  "quantity_built" text,
  "power_horsepower" real,
  "max_speed_km_h" real
);
SELECT SUM("max_speed_km_h") FROM "diesel_multiple_units" WHERE "quantity_built"='8+4' AND "year_built"<1971;