[
  "First find how much money Ms Tan brought to school: $3 (spent) + $6 (left) = $9.",
  "This $9 is 3/7 of her weekly pocket money. So 3/7 of total = 9.",
  "Total = 9 divided by (3/7) = 9 * (7/3) = 63/3 = $21."
]