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_47 (system VARCHAR, internal_hard_disk VARCHAR, s_100_slots VARCHAR, year_introduced VARCHAR)

### Question ###
Which system introduced earlier than 1981 has an internal hard disk of 11 megabytes and less than 21 s-100 slots?

### Accurate SQL ###

SELECT system FROM table_name_47 WHERE s_100_slots < 21 AND year_introduced < 1981 AND internal_hard_disk = "11 megabytes"