Subtract till 20

Calculate:

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

13	-	8	=
7	-	7	=
20	-	3	=
15	-	9	=
10	-	6	=
14	-	6	=
18	-	5	=
18	-	3	=
12	-	4	=
20	-	6	=