Suppose you are starting an office-cleaning service you have spent $315 on equipment to clean each office, you use $4 worth of supplies you charge $25 per office how many offices must you clean to break even

15 offices should be cleaned to break even.

Break even is when your revenue is equal to your expenses resulting to zero profit.

Let x be the number of cleaned offices.

Revenue = 25x
Expenses = Fixed Expense + Variable Expense = 315 + 4x

25x           =    315 + 4x
25x - 4x    =    315
         21x  =    315
 21x / 21   = 315 / 21
            x   = 15 offices.

To check:
25 (15)   = 315 + 4 (15)
      375  = 315 + 60
      375  = 375

Profit = Revenue - Expenses
Profit = 12x - (315 + 4x)
Profit = 12(15) - 315 - 4(15)
Profit = 375 - 375
Profit = 0