Subtract till 20

Calculate:

Number of examples:
Numerical difficulty:
Range of values:
Multiplicity:

17	-	8	=
8	-	7	=
15	-	2	=
17	-	2	=
17	-	6	=
10	-	7	=
19	-	5	=
18	-	7	=
16	-	9	=
10	-	6	=