CREATE TABLE table_name_68 (quantity_preserved VARCHAR2, manufacturer VARCHAR2, class VARCHAR2)

How many did alco -schenectady preserve of class b-3?
SELECT quantity_preserved FROM table_name_68 WHERE manufacturer = "alco -schenectady" AND class = "b-3"
SELECT "quantity_preserved" FROM "table_name_68" WHERE "alco -schenectady" = "manufacturer" AND "b-3" = "class"