Subnet Masks to Remember and their decimal notations

I find that it's really time wasting something to derive the decimal notations from the / notation. In exam, its time-contraint. So better to remember the common ones:

/ Notation <-> Decimal (last octet)

30 <-> 252
29 <-> 248
28 <-> 240
27 <-> 224
26 <-> 192
25 <-> 128