Brody plans to use grass seed in the backyard of his newly built home. His backyard is a 36’ x 40’ rectangular plot but he has installed a 12’ x 24’ in ground pool. If each bag of grass seed covers 375 ft.² how many bags will he need to purchase
The total area of the backyard is 36 * 40 = <<36*40=1440>>1440 ft².
The area of the pool is 12 * 24 = <<12*24=288>>288 ft².
The remaining area for grass seed is 1440 - 288 = <<1440-288=1152>>1152 ft².
Brody will need to purchase 1152 / 375 = <<1152/375=3.072>>3.072 bags of grass seed.
Since he cannot purchase a fraction of a bag, he will need to purchase 4 bags of grass seed. Answer: \boxed{4}.