Adding till 20

Calculate:

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

13	+	7	=
9	+	7	=
4	+	4	=
5	+	4	=
7	+	7	=
6	+	3	=
5	+	9	=
11	+	7	=
2	+	2	=
1	+	2	=