Adding till 20

Calculate:

Number of examples:
Numerical difficulty:
Range of values:
Multiplicity:

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