Subtract till 20

Calculate:

Number of examples:
Numerical difficulty:
Range of values:
Multiplicity:

19	-	4	=
18	-	9	=
19	-	7	=
9	-	9	=
5	-	3	=
20	-	2	=
14	-	4	=
9	-	7	=
17	-	4	=
7	-	6	=