CREATE TABLE table_13921 ("Class" TEXT, "Railway number ( s ) " TEXT, "Quantity" FLOAT, "Year ( s )  of manufacture" TEXT, "Type" TEXT)

Which Year(s) of manufacture has a Type of 1b n2, and a Quantity of 10?
SELECT "Year(s) of manufacture" FROM table_13921 WHERE "Type" = '1b n2' AND "Quantity" = '10'
SELECT "Year(s) of manufacture" FROM "table_13921" WHERE "Quantity" = '10' AND "Type" = '1b n2'