They only help when you:
A: either have enough money to buy them when needed or bought them in advance. (For me it was not the case when I needed to free the hands to work for said money.)
And
B: your hands are free. If not, good luck guessing a number, the tips when choosing a wrong number are helpful, but there still is a good chance to waste a Key for just guessing bad.
Not necessary because if you do 70 if it's too low do 30 then you can easily do it, even better if you "Quick Save" before start guessing because it's the same number and you can keep trying then if you are close but not sure just "Quick Load" After that, you can guess.
The money you can get is quite fast because if you work at the mines in 1 day you can get around 3 credits if you want mastery/level you can either fight with the mine girl or fight guards at the farm you can get items, exp, money if you run out of energy you can always drink an energy drink if it's still too hard just do a "Quick Save Fight", the best way to fight is to shove them to the ground then bite because it deals the highest damage and causes bleeding if they manage to get up just Quick save before shoving so if you fail u can load.
