How many players, in games played in 1990, achieved 50% or less of oRebounds than dRebounds.
in 1990 refers to season_id = 1990; 50% or less of oRebounds than dRebounds refers to o_rebounds < = multiply(d_rebounds, 0.5)
SELECT COUNT(playerID) FROM players_teams WHERE CAST(oRebounds AS REAL) * 100 / dRebounds <= 50 AND year = 1990