估算:23×10≈ 49×10≈ 28×30≈ 38×10≈ 41×10≈ 42×10≈
8-2= 8-5= 18-6= 12-1= 7-5= 12-2=
17-5= 17-3= 17-2= 17-1= 17-6= 17-4=
16-1= 16-3= 16-5= 16-4= 16-0= 16-2=
17-2= 17-4= 18-6= 12-1= 7-5= 12-2=
9-3= 9-5= 18-2= 11-1= 9-6= 9-1=