[
  "Work backwards from the $15 left. Since 1/4 of the remainder was spent on the toy, the $15 left is 3/4 of the remainder after the watch.",
  "Remainder after watch = $15 / 3 x 4 = $20. This $20 is 2/3 of total (since 1/3 spent on watch). Total = $20 / 2 x 3 = $30.",
  "Spent on watch = 1/3 x $30 = $10. Answer: 10."
]