(1)4分=0.04元;
(2)3角5分=0.35元;
(3)40厘米=0.4米;
(4)250千克=0.25吨;
(5)18平方分米=0.18平方米;
(6)18秒=0.3分.
故答案为:0.04,0.35,0.4,0.25,0.18,0.3.