Table schema:
CREATE TABLE table_name_43 (main_span_feet VARCHAR, main_span_metres VARCHAR, rank VARCHAR, year_opened VARCHAR, country VARCHAR)

Question: What is the main span feet from opening year of 1936 in the United States with a rank greater than 47 and 421 main span metres?
SQL: SELECT main_span_feet FROM table_name_43 WHERE year_opened = 1936 AND country = "united states" AND rank > 47 AND main_span_metres = "421"