SELECT COUNT("Acres") FROM table_41788 WHERE "Landfill" = 'sai tso wan' AND "Opened" < '1978'
What is the total number of acres at Sai Tso Wan, opening before 1978?
CREATE TABLE table_41788 (
    "Landfill" text,
    "Location" text,
    "Acres" real,
    "Opened" real,
    "Capacity" text,
    "Status" text,
    "Rehab Period" text
)