[
  "Convert 1 and 1/2 litres to an improper fraction with denominator 20: it becomes 30/20.",
  "Convert 2/5 to twentieths: 8/20. The amount given away is already 9/20.",
  "Subtract both from the total: 30/20 minus 8/20 minus 9/20 = 13/20 litres."
]