Subtract till 20

Calculate:

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

9	-	9	=
6	-	6	=
9	-	3	=
10	-	2	=
12	-	6	=
15	-	6	=
5	-	3	=
15	-	3	=
11	-	9	=
16	-	8	=