Subtract till 20

Calculate:

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

12	-	9	=
19	-	7	=
19	-	2	=
15	-	7	=
19	-	3	=
18	-	6	=
10	-	8	=
20	-	9	=
18	-	4	=
18	-	9	=