Subtract till 20

Calculate:

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

16	-	6	=
19	-	5	=
8	-	7	=
5	-	2	=
8	-	3	=
20	-	5	=
13	-	4	=
6	-	6	=
20	-	2	=
13	-	4	=