a=1时,A的个数：{15},{125},{135},{145},{1235},{1245},{1345},当a=2时,A的个数：{24},{124},{234},{245},{1234},{1245}与a=1时有重复,舍去.{2345},a=3时,A的个数：{3},{13},{23},{34},{35},{123},{134},{135},{234}有重复,舍去.{235},{345},{1234}重复,舍去.{1235},{2345}重复,舍去.最后还有一个{12345}.a=4,与a=1重复,a=5,与a=1重复.所以A的组合一共25种.