Question 104816
Establish a variable to represent number of offices cleaned.  Let x = number of offices cleaned, then to determine your break even point you must use an inequality.  If I'm reading the problem correctly you earn $25 for each office cleaned but you spend $4 to clean each office, therefore your net  is $21 per office.

21x >=$315

x>= 315 / 21 => gives you the minimum number of offices to clean.