Subtract till 20

Calculate:

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

45	-	44	=
20	-	12	=
37	-	24	=
56	-	29	=
60	-	5	=
83	-	22	=
54	-	42	=
21	-	3	=
63	-	26	=
27	-	6	=