CREATE TABLE table_64473 ("Week" FLOAT, "Date" TEXT, "Opponent" TEXT, "Result" TEXT, "Attendance" FLOAT)

How many people attended the September 21, 1980 game?
SELECT AVG("Attendance") FROM table_64473 WHERE "Date" = 'september 21, 1980'
SELECT AVG("Attendance") FROM "table_64473" WHERE "Date" = 'september 21, 1980'