[
  "Round each: 12,640 -> 13,000 (hundreds 6 > 5); 8,470 -> 8,000 (hundreds 4 < 5); 9,810 -> 10,000 (hundreds 8 > 5).",
  "Add: 13,000 + 8,000 = 21,000.",
  "21,000 + 10,000 = 31,000."
]