human
Considering the provided database schema and associated query, produce SQL code to retrieve the answer to the query. 
###Database Schema: Rank, Main span metres, Main span feet, Year opened, Country
###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?
gpt
SELECT Main span feet FROM table WHERE Year opened = 1936 AND Country = united states AND Rank > 47 AND Main span metres = 421