Subtract till 20

Calculate:

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

6	-	2	=
8	-	2	=
16	-	5	=
18	-	9	=
12	-	6	=
20	-	7	=
6	-	2	=
20	-	7	=
6	-	3	=
13	-	8	=