what is the average foreign imports s tone when the total s tone is 3,157,247 and the foreign exports s ton is less than 358,493?
CREATE TABLE "tonnage_for_port_of_anchorage" (
  "year" real,
  "u_s_rank" real,
  "total_s_ton" real,
  "domestic_s_ton" real,
  "foreign_total_s_ton" real,
  "foreign_imports_s_ton" real,
  "foreign_exports_s_ton" real
);
SELECT AVG("foreign_imports_s_ton") FROM "tonnage_for_port_of_anchorage" WHERE "total_s_ton"='3,157,247' AND "foreign_exports_s_ton"<'358,493';