CREATE TABLE table_79598 ("Class" TEXT, "Railway number ( s ) " TEXT, "Quantity" FLOAT, "Year ( s )  of manufacture" TEXT, "Axle arrangement  (  UIC  ) " TEXT)

What year was the b n2t axle arrangement, which has a quantity of 31, manufactured?
SELECT "Year(s) of manufacture" FROM table_79598 WHERE "Quantity" = '31' AND "Axle arrangement ( UIC )" = 'b n2t'
SELECT "Year(s) of manufacture" FROM "table_79598" WHERE "Axle arrangement ( UIC )" = 'b n2t' AND "Quantity" = '31'