Adding till 20

Calculate:

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

9	+	10	=
8	+	9	=
2	+	3	=
8	+	7	=
10	+	6	=
4	+	6	=
7	+	5	=
3	+	3	=
5	+	1	=
4	+	1	=