What is the density of the common with an area of 20.4 km^2?
CREATE TABLE table_72561 (
    "Rank" text,
    "Common of" text,
    "Population" real,
    "Area (km 2 )" text,
    "Density (inhabitants/km 2 )" text,
    "Altitude (mslm)" real
)
SELECT "Density (inhabitants/km 2 )" FROM table_72561 WHERE "Area (km 2 )" = '20.4'
Below are sql tables schemas paired with instruction that describes a task. Using valid SQLite, write a response that appropriately completes the request for the provided tables. ### Instruction: What is the density of the common with an area of 20.4 km^2? ### Input: CREATE TABLE table_72561 (
    "Rank" text,
    "Common of" text,
    "Population" real,
    "Area (km 2 )" text,
    "Density (inhabitants/km 2 )" text,
    "Altitude (mslm)" real
) ### Response: SELECT "Density (inhabitants/km 2 )" FROM table_72561 WHERE "Area (km 2 )" = '20.4'