Subtract till 20

Calculate:

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

9	-	9	=
6	-	2	=
19	-	5	=
13	-	9	=
15	-	6	=
10	-	8	=
13	-	2	=
12	-	4	=
2	-	2	=
5	-	3	=