(Total rental cost)/20,000 = rental cost per brick.
(Rental cost per brick)*1000 = rental cost per 1000 brick
Or, if you prefer,
(Total Rental Cost)/20 = rental cost per 1000 brick
(Rental cost per brick)*1000 = rental cost per 1000 brick
Or, if you prefer,
(Total Rental Cost)/20 = rental cost per 1000 brick