[
  "After Week 1: 70% left. Week 2 uses 25% of 70% = 17.5%. After Week 2: 52.5% left.",
  "Week 3 uses 40% of 52.5% = 21%. After Week 3: 31.5% left = 6.3 litres.",
  "Original = 6.3 / 0.315 = 20 litres."
]