the milk becomes a variable expense because it doesn't have a fixed amount. it could be 2 or 3 or 4 or 1 depending on the number of persons having breakfast everyday over the course of the week but not 5.
if 5 milks are purchased day-in, day-out, the milk would be afixed expense but that isnt the case here.
the term variable expense ffrom the name means an expense that changes or varies from time to time
