20以内退位减法总共也没有500道呀,怎么可能不重复呢?11-9 12-9 13-9 14-9 15-9 16-9 17-9 18-9 11-8 12-8 13-8 14-8 15-8 16-8 17-811-7 12-7 13-7 14-7 15-7 16-711-6 12-6 13-6 14-6 15-6 11-5 12-5 13-5 14-511-4...