20以内退位减法(1)
11-7= 13-5= 14-9= 12-8=
11-9= 12-9= 12-7= 11-3=
12-3= 16-7= 15-6= 13-9=
17-9= 14-8= 15-9= 17-8=
13-8= 12-6= 14-7= 11-6=