[
  "After selling s kg, 450 - s remains. 20% of that is given away, so 80% remains: 0.80(450 - s) = 180.",
  "Alternatively: sold s, gave 20% of remainder = 0.20(450-s). Left = (450-s) - 0.20(450-s) = 0.80(450-s).",
  "0.80(450 - s) = 180, so 450 - s = 225, s = 225. Percentage sold = 225/450 * 100% = 50%."
]