Adding till 20

Calculate:

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

4	+	3	=
1	+	2	=
4	+	2	=
5	+	6	=
3	+	4	=
2	+	3	=
4	+	1	=
8	+	9	=
6	+	6	=
10	+	10	=