Subtract till 20

Calculate:

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

15	-	8	=
19	-	8	=
15	-	7	=
17	-	9	=
20	-	8	=
19	-	2	=
20	-	7	=
9	-	8	=
16	-	4	=
12	-	3	=