Subtract till 20

Calculate:

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

16	-	5	=
18	-	3	=
11	-	3	=
6	-	5	=
10	-	5	=
16	-	5	=
8	-	4	=
17	-	8	=
6	-	4	=
10	-	6	=