Proměnná v SQL -> ERROR PLS-00103:Narazil na symbol SELECT

k navázání proměnné

DECLARE
    ESTACION_ID NUMBER(6) :=3;
    v_sql   varchar2(32767);
BEGIN
    v_sql   := 'SELECT 100*TOTAL_TAX_AMB1/(SELECT COUNT(*) FROM ITV2 WHERE ITV2.C_RESULTADO=1 AND ITV2.C_ESTACION_ID= :1) AS TAXIS_Y_AMBULANCIAS_PRIMERA
             ...'
    EXECUTE IMMEDIATE v_sql INTO ... USING ESTACION_ID;
END;