Subtract till 20

Calculate:

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

11	-	3	=
11	-	9	=
20	-	3	=
12	-	9	=
15	-	4	=
14	-	9	=
9	-	7	=
20	-	2	=
7	-	7	=
17	-	4	=