Subtract till 20

Calculate:

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

19	-	2	=
11	-	9	=
14	-	7	=
13	-	6	=
16	-	8	=
10	-	7	=
7	-	4	=
14	-	8	=
15	-	3	=
11	-	9	=