[
  "Work backwards from the file cost of $4. The $4 is 1/2 of the remainder after food, so the remainder is double the file cost.",
  "Remainder after food = $4 x 2 = $8. This $8 is 2/3 of total (since 1/3 was spent on food). Total at first = $8 / 2 x 3 = $12.",
  "Answer: $12."
]