Subtract till 20

Calculate:

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

20	-	7	=
13	-	6	=
18	-	9	=
6	-	4	=
9	-	3	=
10	-	5	=
14	-	8	=
15	-	6	=
11	-	3	=
20	-	2	=