Can you use either פחות or מינוס here?
no, in this situation u have to use in מינוס beacuse it tells u that minus one is negative,
but if u say one minus one u need to use פחות
why is the 'one' female here?
We use the feminine form for abstract numbers.
what makes a number abstract?
A number you "just count" with (one, two, three...), as opposed to counting nouns (two apples...)
mínus achát chelkéy mínus achát shavé achát.