Subtract till 20

Calculate:

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

16	-	6	=
12	-	8	=
11	-	7	=
8	-	7	=
13	-	2	=
9	-	9	=
9	-	8	=
15	-	9	=
16	-	8	=
7	-	2	=