细心算一算。

0+4=          10+6=          3+8=          16-5=

19-5=          10-7=          2+9=          4+13=

18-4=          4+9=          18-6=          9+3=

17-6=          9+3=          7+5=          6+4=

6+7-2=          15-2-3=          17-7+3=          9-2+8=