question: Which system introduced earlier than 1981 has an internal hard disk of 11 megabytes and less than 21 s-100 slots?
answer: SELECT system FROM table_name_47 WHERE s_100_slots < 21 AND year_introduced < 1981 AND internal_hard_disk = "11 megabytes"
context: CREATE TABLE table_name_47 (system VARCHAR, internal_hard_disk VARCHAR, s_100_slots VARCHAR, year_introduced VARCHAR)