Adding till 20

Calculate:

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

10	+	10	=
15	+	5	=
3	+	4	=
2	+	4	=
8	+	6	=
5	+	5	=
4	+	2	=
7	+	11	=
4	+	5	=
5	+	4	=