Below is the list of tables from the database along with their columns and datatype. Need to generate an accurate SQL query for the question.
Only use the columns present in the respective table. Only use the columns which are required to generate the SQL query. Only use table joins if required and not otherwise.

### Tables ###
TABLE: CREATE TABLE table_name_68 (quantity_preserved VARCHAR, manufacturer VARCHAR, class VARCHAR)

### Question ###
How many did alco -schenectady preserve of class b-3?

### Accurate SQL ###

SELECT quantity_preserved FROM table_name_68 WHERE manufacturer = "alco -schenectady" AND class = "b-3"