CREATE TABLE table_15707 ("Year" FLOAT, "Entrant" TEXT, "Chassis" TEXT, "Engine" TEXT, "Tyres" TEXT, "Points" FLOAT)

What year was the brabham bt46 bt48 bt49 Chassis common?
SELECT AVG("Year") FROM table_15707 WHERE "Chassis" = 'brabham bt46 bt48 bt49'
SELECT AVG("Year") FROM "table_15707" WHERE "Chassis" = 'brabham bt46 bt48 bt49'