Adding till 20

Calculate:

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

1	+	4	=
8	+	8	=
6	+	11	=
12	+	8	=
2	+	3	=
2	+	4	=
7	+	7	=
11	+	9	=
7	+	6	=
4	+	2	=