[
  "Draw a bar model: the number of girls stays the same (9). At first, boys:girls = 10:3. After some boys leave, the ratio becomes 3:1.",
  "3 units = 9 girls, so 1 unit = 9 / 3 = 3. Boys at first = 10 x 3 = 30. Boys after = 3 x 9 = 27.",
  "30 - 27 = 3. Answer: 3 boys must leave the class."
]