I think the only confusing part of it is the sentence > The number of phones that she has left to fix at the end of each day can be estimated with the equation

I think the only confusing part of it is the sentence > The number of phones that she has left to fix at the end of each day can be estimated with the equation "Has left to fix" could be misinterpreted as meaning "she has to fix this many," instead of simply "how many remain." If you understand that it's just saying "P is how many remain", then it's clear that the formula is simply saying that she started with 108, and she fixes 23 per day.