Subtract till 20

Calculate:

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

11	-	7	=
8	-	6	=
15	-	4	=
19	-	6	=
19	-	5	=
13	-	2	=
19	-	5	=
16	-	5	=
9	-	8	=
19	-	6	=