Que sistema introduzido antes de 1981 tem um disco rígido interno de 11 megabytes e menos de 21 slots s-100?
CREATE TABLE table_name_47 (system VARCHAR, internal_hard_disk VARCHAR, s_100_slots VARCHAR, year_introduced VARCHAR)
SELECT system FROM table_name_47 WHERE s_100_slots < 21 AND year_introduced < 1981 AND internal_hard_disk = "11 megabytes"