What is the % (1960) of the county with a % (2040) of 3.4?
CREATE TABLE "historical_population" (
  "rank" text,
  "county" text,
  "pct_1960" real,
  "pct_2000" real,
  "pct_2040" real
);
SELECT COUNT("pct_1960") FROM "historical_population" WHERE "pct_2040"=3.4;