[
  "After day 1: 80% of original remains = 0.80x. After adding 10L: 0.80x + 10.",
  "After using 25%, 75% remains: 0.75(0.80x + 10) = 0.60x + 7.5 = 45.",
  "Solve: 0.60x = 37.5, x = 62.5 litres."
]