Subtract till 100

Calculate:

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

57	-	30	=
64	-	23	=
97	-	8	=
98	-	42	=
56	-	28	=
60	-	28	=
35	-	18	=
90	-	21	=
91	-	22	=
70	-	29	=