Adding till 20

Calculate:

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

4	+	4	=
2	+	1	=
5	+	3	=
9	+	6	=
7	+	3	=
5	+	7	=
6	+	7	=
5	+	5	=
11	+	6	=
7	+	11	=