[
  "Draw a bar model: At first, boys:girls = 1:3. Let boys = 1u, girls = 3u. After 4 boys and 4 girls join, boys:girls = 1:2.",
  "Equation: (u + 4) / (3u + 4) = 1/2. Cross-multiply: 2(u + 4) = 3u + 4. So 2u + 8 = 3u + 4, which gives u = 4.",
  "Total at first = 1u + 3u = 4u = 4 x 4 = 16. Answer: 16 pupils."
]