For how many years was the urban population 57%?
CREATE TABLE table_20935 (
    "Year (January)" real,
    "Population (000)" real,
    "Rural, %" real,
    "Urban, %" real,
    "Source" text
)
SELECT COUNT("Source") FROM table_20935 WHERE "Urban, %" = '57'
Below are sql tables schemas paired with instruction that describes a task. Using valid SQLite, write a response that appropriately completes the request for the provided tables. ### Instruction: For how many years was the urban population 57%? ### Input: CREATE TABLE table_20935 (
    "Year (January)" real,
    "Population (000)" real,
    "Rural, %" real,
    "Urban, %" real,
    "Source" text
) ### Response: SELECT COUNT("Source") FROM table_20935 WHERE "Urban, %" = '57'