How many number Builts had unit numbers of 375301-310?
CREATE TABLE "fleet_details" (
  "class" text,
  "type" text,
  "operator" text,
  "no_built" real,
  "year_built" text,
  "cars_per_set" real,
  "unit_nos" text
);
SELECT SUM("no_built") FROM "fleet_details" WHERE "unit_nos"='375301-310';