Calculate the percentage of countries that belong to the American region.
the American region refers to r_name = 'America'; percentage = divide(count(n_name where r_name = 'America'), count(n_name)) * 100%
SELECT CAST(SUM(IIF(T1.r_name = 'America', 1, 0)) AS REAL) * 100 / COUNT(T2.n_name) FROM region AS T1 INNER JOIN nation AS T2 ON T1.r_regionkey = T2.n_regionkey