Subtract till 20

Calculate:

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

10	-	4	=
11	-	9	=
15	-	6	=
14	-	9	=
16	-	6	=
10	-	3	=
7	-	5	=
10	-	3	=
7	-	7	=
9	-	6	=