The son is 3 units and the daughter is 1 unit. Both totals share Maria, so subtracting them leaves son − daughter = 2 units.<br><br>[[model:questions/solutions/q4180.png]]<br><br>• (Maria + son) − (Maria + daughter) → 100 − 70 = 30<br>• Son − daughter = 2 units → 30<br>• 1 unit (daughter) → 30 ÷ 2 = 15; son = 45<br>• Maria → 70 − 15 = 55 kg<br><br>Maria weighs 55 kg.<br>Check: Maria 55 + son 45 = 100; Maria 55 + daughter 15 = 70; 45 = 3 × 15. ✓