Of all the countries in which English is spoken, what percentage has English as their only language?
Percentage = [count(countries 100% English) / count(countries English)] * 100%
SELECT CAST(SUM(CASE WHEN T2.Percentage = 100 THEN 1 ELSE 0 END) AS REAL) * 100 / COUNT(T1.Name) FROM country AS T1 INNER JOIN language AS T2 ON T1.Code = T2.Country WHERE T2.Name = 'English'