CREATE TABLE table_name_47 (system VARCHAR, internal_hard_disk VARCHAR, s_100_slots VARCHAR, year_introduced VARCHAR)

Which system introduced earlier than 1981 has an internal hard disk of 11 megabytes and less than 21 s-100 slots?
SELECT system FROM table_name_47 WHERE s_100_slots < 21 AND year_introduced < 1981 AND internal_hard_disk = "11 megabytes"
SELECT "system" FROM "table_name_47" WHERE "11 megabytes" = "internal_hard_disk" AND "s_100_slots" < 21 AND "year_introduced" < 1981