[
  "Draw a bar model: 2/7 of boys = 4/5 of girls. Multiply both sides by 35 (LCM of 7 and 5): 10B = 28G. Simplify to 5B = 14G, so boys : girls = 14 : 5.",
  "Difference = 14 units - 5 units = 9 units = 18. So 1 unit = 18 / 9 = 2.",
  "Boys = 14 x 2 = 28. Answer: 28 boys."
]