user: 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?
Columns: ['Rank', 'Main span metres', 'Main span feet', 'Year opened', 'Country']
Types: ['real', 'text', 'text', 'real', 'text']
Sample row: [1.0, '1,991', '6,532', 1998.0, 'Japan']
SQL:

assistant: SELECT "Main span feet" FROM "Table" WHERE "Year opened" = 1936.0 AND "Country" = 'United States' AND "Rank" > 47.0 AND "Main span metres" = '421';