[
  "Draw a bar model: set up Before and After with units. At first, Adults = 4u, Children = 5u.",
  "After: Adults = 4u + 6, Children = 5u - 6. Ratio is 7 : 2, so 2(4u + 6) = 7(5u - 6). Solve: 8u + 12 = 35u - 42, 27u = 54, u = 2.",
  "End total = (4x2 + 6) + (5x2 - 6) = 14 + 4 = 18 passengers."
]