Subtract till 20

Calculate:

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

8	-	4	=
15	-	5	=
9	-	5	=
19	-	3	=
17	-	4	=
10	-	9	=
8	-	8	=
10	-	7	=
16	-	4	=
18	-	5	=