Subtract till 20

Calculate:

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

7	-	5	=
10	-	8	=
18	-	5	=
20	-	8	=
17	-	9	=
16	-	8	=
5	-	5	=
15	-	4	=
9	-	7	=
15	-	8	=