CREATE TABLE table_73265 ("Gas name" TEXT, "Chemical formula" TEXT, "Lifetime  ( years ) " TEXT, "20-yr" TEXT, "100-yr" TEXT, "500-yr" TEXT)

What is the 20 year for Nitrous Oxide?
SELECT "20-yr" FROM table_73265 WHERE "Gas name" = 'Nitrous oxide'
SELECT "20-yr" FROM "table_73265" WHERE "Gas name" = 'Nitrous oxide'