Subtract till 100

Calculate:

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

57	-	27	=
68	-	36	=
66	-	37	=
48	-	26	=
26	-	16	=
89	-	4	=
60	-	26	=
9	-	3	=
80	-	19	=
77	-	21	=