Twice Bill equals thrice Blair, so Bill : Blair = 3 : 2. Bill = 3 units, Blair = 2 units, and the 1-unit difference is the 17 kg Bill is heavier.<br><br>[[model:questions/solutions/q4198.png]]<br><br>• 2 × Bill = 3 × Blair ⇒ Bill : Blair = 3 : 2<br>• Difference → 3 units − 2 units = 1 unit → 17 kg<br>• Total → 3 + 2 = 5 units → 5 × 17 = 85 kg<br><br>The total mass of the two children is 85 kg.<br>Check: Bill 51, Blair 34; 2 × 51 = 102 = 3 × 34; 51 − 34 = 17. ✓