Question: What is the average percentage of agriculture of GDP in countries on the African Continent?
SQL: SELECT AVG(T4.Agriculture) FROM continent AS T1 INNER JOIN encompasses AS T2 ON T1.Name = T2.Continent INNER JOIN country AS T3 ON T3.Code = T2.Country INNER JOIN economy AS T4 ON T4.Country = T3.Code WHERE T1.Name = 'Africa'