With a number of 30, what would be listed for previous number(s)?
CREATE TABLE "great_northern_section" (
  "number" text,
  "previous_number_s" real,
  "previous_class" text,
  "converted" real,
  "withdrawn" real,
  "disposal" text
);
SELECT "previous_number_s" FROM "great_northern_section" WHERE "number"='30';