CREATE TABLE table_2980 ("Series" FLOAT, "Start date" TEXT, "End date" TEXT, "Episodes" FLOAT, "Average viewers  ( millions ) " TEXT)

How many different end dates are there for the series seen by 4.2 million people?
SELECT COUNT("End date") FROM table_2980 WHERE "Average viewers (millions)" = '4.2'
SELECT COUNT("End date") FROM "table_2980" WHERE "Average viewers (millions)" = '4.2'