CREATE TABLE table_5680 ("Gauge" TEXT, "Railway" TEXT, "Class" TEXT, "Works no." TEXT, "Year" TEXT, "Builder" TEXT)

Which works number has a class of 15th and year of 1940?
SELECT "Works no." FROM table_5680 WHERE "Class" = '15th' AND "Year" = '1940'
SELECT "Works no." FROM "table_5680" WHERE "Class" = '15th' AND "Year" = '1940'