You are a civil engineer designing a bridge. The walkway needs to be made of woodes planks. You are able to use either Saka spruce planks (which weigh 3 pounds cach) wood planks which weight 4 pounds each or a combination of both The total weight of the planks must be between 600 and 900 pounds in order to meet safety code " Sitka spruce planks cout RM25 beach and basswood planks cost RM3 75 each, how many of each plank should you use to minimize cost while still meeting building code?