[
  "Draw a bar model: the number of boys does not change. Only girls are transferred out.",
  "At first: Boys = 1u, Girls = 2u. After: Boys = 1u, Girls = 2u - 3. Ratio 3 : 5 means 5(1u) = 3(2u - 3). Solve: 5u = 6u - 9, u = 9.",
  "There were 9 boys in the class."
]