user: Question: For how many years was the urban population 57%?
Columns: ['Year (January)', 'Population (000)', 'Rural, %', 'Urban, %', 'Source']
Types: ['real', 'real', 'real', 'real', 'text']
Sample row: [1939.0, 6081.0, 72.0, 28.0, 'census']
SQL:

assistant: SELECT COUNT("Source") FROM "Table" WHERE "Urban, %" = 57.0;