[
  "Work backwards from the 450 ml left. At each step, the amount kept is the complement of the fraction consumed or given away.",
  "450 ml = 3/4 after giving to friend (1/4 given). Before giving: 450 / 3 x 4 = 600 ml. 600 ml = 2/3 after afternoon (1/3 drunk). Before afternoon: 600 / 2 x 3 = 900 ml. 900 ml = 3/5 after morning (2/5 drunk).",
  "Amount at first = 900 / 3 x 5 = 1500 ml. Answer: 1500 ml (option C)."
]