Adding till 20

Calculate:

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

9	+	7	=
6	+	12	=
2	+	2	=
6	+	13	=
1	+	2	=
6	+	10	=
4	+	4	=
1	+	6	=
13	+	3	=
11	+	2	=