[
  "Work backwards from the file cost of $4. The $4 is 1/4 of the remainder after food, so multiply to find the whole remainder.",
  "Remainder after food = $4 x 4 = $16. This $16 is 8/9 of total money (since 1/9 was spent on food). Total money = $16 / 8 x 9 = $18.",
  "Spent on food = 1/9 x $18 = $2. Answer: 2."
]