You are the operational manager of an over-head bridge project. The walkway needs to be made of wooden planks. According to the contract you are allowed to use either Kumbuk plank which weigh 3 kilograms each or Wal-Dhel plank which weigh 4.5 kilograms each. A minimum of 100 Kumbuk planks are required for the edges of the walk-way. The total weight of the planks must be between 660 and 975 kilograms in order to meet the safety code. If kumbuk planks cost LKR 5900 each and Wal-Dhel planks cost LKR 5100 each, how many of each plank should be used to minimize the cost while meeting the building safety code?