Subtract till 20

Calculate:

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

9	-	5	=
8	-	4	=
8	-	6	=
12	-	7	=
19	-	7	=
12	-	2	=
20	-	6	=
19	-	2	=
7	-	4	=
5	-	4	=