Adding till 20

Calculate:

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

9	+	10	=
4	+	2	=
4	+	7	=
6	+	9	=
12	+	7	=
8	+	9	=
8	+	8	=
3	+	4	=
7	+	8	=
11	+	9	=