Subtract till 20

Calculate:

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

11	-	8	=
18	-	6	=
19	-	6	=
13	-	6	=
5	-	2	=
17	-	2	=
3	-	2	=
8	-	6	=
12	-	9	=
20	-	3	=