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_48787 ( "System" text, "Year Introduced" real, "S-100 slots" real, "Internal Floppy Disk" text, "Internal Hard Disk" text )

### 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_48787 WHERE "S-100 slots" < '21' AND "Year Introduced" < '1981' AND "Internal Hard Disk" = '11 megabytes'