Boys stay at 1u while 3 girls leave. After the transfer, Boys(1u) : Girls(2u−3) = 3:5. Cross-multiply to find u.<br><br>[[model:questions/solutions/q4524.png]]<br><br>• Before: Boys = 1u, Girls = 2u<br>• After: Boys = 1u, Girls = 2u − 3<br>• New ratio → 1u : (2u − 3) = 3 : 5<br>• Cross-multiply → 5 × 1u = 3(2u − 3)<br>• 5u = 6u − 9 → u = 9; Boys = 9<br><br>There were 9 boys in the class.<br>Check: Girls at first = 18, after 3 leave = 15; ratio 9:15 = 3:5. ✓