Write SQL query to solve given problem: Please calculate the percentage of Sub-Saharan African countries which are in the Special trade system.. Keep the solution inside sql tag ```sql [SQL-Query] ```
SELECT  CAST(SUM(CASE WHEN Region = 'Sub-Saharan Africa' THEN 1 ELSE 0 END) AS REAL) * 100 / COUNT(CountryCode) FROM Country WHERE SystemOfTrade = 'Special trade system'