O1 the relationship between the number of $4 lunches you buy with a $100 school lunch card and the money remaining on the card

Every time you buy a $4 lunch, $4 is subtracted from the remaining balance on your school lunch card.  If x represents the # of lunches you've bought, then

the $ remaining on the card = $100 - ($4)x.  If x=0 (no lunches), you have $100 on the card; if x =25, you have 0 remaining.