[
  "Work backwards from the file cost of $4. The $4 is 2/5 of the remainder after food. Find one-fifth first, then the whole remainder.",
  "1/5 of remainder = $4 / 2 = $2. Remainder after food = $2 x 5 = $10. This $10 is 2/3 of total (since 1/3 spent on food). Total = $10 / 2 x 3 = $15. Food = 1/3 x $15 = $5.",
  "Money left = $15 - $5 - $4 = $6. Answer: 6."
]