"Impacted" is it now ? Just how do you measure or describe 'impact' mathematically ? What are its physical dimensions ?
The credit balance in her account decreases by $3, or the debt balance in it decreases by $3, for each time she has charged her lunch during that period of time. It may also be a little bit more than $3 to cover sales tax and the cost of whatever she has taken to neutralize the charge imparted by a lunch in the company cafeteria.
To make it look smart and technical, we might write
B = B₀ - 3L
where
B = the balance in Valerie's lunch account (unit is $)
B₀ = the balance at the beginning of the period of time (unit is $)
L = the number of lunches she has charged since the beginning of the period of time (no unit)
Note: I would take with a grain of salt any data provided by Valerie and not independently corroborated. I have never trusted anybody who can't spell his or her own name properly.