[
  "Base area = 30 x 25 = 750 cm^2. Initial oil = 750 x 8 = 6000 cm^3.",
  "Total oil after adding 9 L (9000 cm^3) = 6000 + 9000 = 15000 cm^3.",
  "Oil height = 15000/750 = 20 cm = 4/5 of tank height. Tank height = 20 x 5/4 = 25 cm."
]