Subtract till 20

Calculate:

The number of examples:
Numerical difficulty:
Range of values:
Multiplicity:

16	-	9	=
16	-	9	=
9	-	4	=
15	-	4	=
18	-	4	=
7	-	2	=
15	-	8	=
9	-	8	=
13	-	8	=
4	-	3	=