What percentage do items under the category of 'Furniture' make up the total number of items ordered that are shipped as standard in the West region?
shipped as standard refers to "Ship Mode" = 'Standard Class'; Region = 'West'; percentage refers to DIVIDE(SUM(Quantity where Category = 'Furniture'), SUM(Quantity)) * 1.0
SELECT CAST(SUM(CASE  WHEN T2.Category = 'Furniture' THEN 1 ELSE 0 END) AS REAL) * 100 / SUM(T1.Quantity) FROM west_superstore AS T1 INNER JOIN product AS T2 ON T1.`Product ID` = T2.`Product ID` WHERE T2.Region = 'West' AND T1.`Ship Mode` = 'Standard Class'