SELECT AVG("Goals against") FROM table_52413 WHERE "Goals for" > '35' AND "Goal Difference" = '20'
What is the average of goals against, where overall goals are more than 35 and the goal difference is 20?
CREATE TABLE table_52413 (
    "Position" real,
    "Club" text,
    "Played" real,
    "Points" text,
    "Wins" real,
    "Draws" real,
    "Losses" real,
    "Goals for" real,
    "Goals against" real,
    "Goal Difference" real
)