You can
put this solution on YOUR website!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.