Girls = 3u = 9, so 1u = 3. Boys = 10u = 30. After some boys leave, the new ratio must be 3:1, so Boys' = 3 × 9 = 27. Boys who left = 30 − 27 = 3.<br><br>[[model:questions/solutions/q4528.png]]<br><br>• Girls = 3 units → 9 girls, so 1 unit → 9 ÷ 3 = 3<br>• Boys at first → 10 × 3 = 30<br>• New ratio Boys' : Girls = 3 : 1<br>• Boys' : 9 = 3 : 1 → Boys' = 3 × 9 = 27<br>• Boys who left → 30 − 27 = 3<br><br>3 boys must leave the class.<br>Check: Before: 30:9 = 10:3. After 3 leave: 27:9 = 3:1. ✓