Subtract till 20

Calculate:

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

10	-	3	=
14	-	2	=
15	-	2	=
20	-	2	=
2	-	2	=
11	-	9	=
19	-	9	=
18	-	5	=
20	-	7	=
19	-	7	=