Each day, Valarie charges her lunch account for her lunch. If the cost of lunch is $3, then by how much has her lunch account been impacted over a period of time?

"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.