Adding till 100

Calculate:

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

9	+	9	=
31	+	26	=
51	+	36	=
24	+	17	=
4	+	4	=
21	+	16	=
40	+	22	=
33	+	32	=
6	+	4	=
20	+	19	=