Among the organizations where headquarters are in the 'USA', what is the percentage of the them are in 'Washington'?
percentage can be computed by [count(City = 'Washington') / count(all cities)] * 100%
SELECT CAST(SUM(CASE WHEN T2.City = 'Washington' THEN 1 ELSE 0 END) AS REAL) * 100 / COUNT(T2.City) FROM country AS T1 INNER JOIN organization AS T2 ON T1.Code = T2.Country WHERE T2.Country = 'USA'