Subtract till 20

Calculate:

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

7	-	5	=
8	-	4	=
9	-	4	=
10	-	4	=
11	-	2	=
17	-	9	=
20	-	6	=
13	-	8	=
5	-	3	=
18	-	4	=