Adding till 20

Calculate:

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

7	+	10	=
1	+	2	=
4	+	1	=
7	+	6	=
4	+	2	=
8	+	5	=
12	+	7	=
8	+	10	=
1	+	4	=
5	+	9	=