CREATE TABLE table_72561 ("Rank" TEXT, "Common of" TEXT, "Population" FLOAT, "Area  ( km 2  ) " TEXT, "Density  ( inhabitants/km 2  ) " TEXT, "Altitude  ( mslm ) " FLOAT)

What is the density of the common with an area of 20.4 km^2?
SELECT "Density (inhabitants/km 2 )" FROM table_72561 WHERE "Area (km 2 )" = '20.4'
SELECT "Density (inhabitants/km 2 )" FROM "table_72561" WHERE "Area (km 2 )" = '20.4'