4-2=
5-1=
4-1=
19
17
13
9
18
20
17的个位上是,十位上是。
13的前面是,后面是。
4+1= 4-2= 1+3= 2+2=
5-3= 2+3= 5-1= 3-1=
1+1= 5-2= 3+2= 5-4=
2+1= 4-3= 1+4= 4-1=
( )