At first, there were 1/3 as many boys as girls. So boys = 1 unit, girls = 3 units. After 4 boys and 4 girls join, the ratio becomes 1:2.<br><br>[[model:questions/solutions/q4377.png]]<br><br>• At first → boys = 1u, girls = 3u<br>• After 4 join each → boys = u + 4, girls = 3u + 4<br>• New ratio → (u + 4) : (3u + 4) = 1 : 2<br>• Cross-multiply → 2(u + 4) = 3u + 4<br>• 2u + 8 = 3u + 4 → u = 4<br>• Total at first → 4u = 4 × 4 = 16<br><br>There were 16 pupils in the class at first.<br>Check: At first 4 boys, 12 girls, ratio 4:12 = 1:3. After 8 boys, 16 girls, ratio 8:16 = 1:2. ✓