The two ages add up to twice the average. Tammie is one unit plus 6; her sister is one unit.<br><br>[[model:questions/solutions/q4672.png]]<br><br>• Total → 2 × 12 = 24<br>• 2 units + 6 → 24<br>• 2 units → 24 − 6 = 18<br>• 1 unit (sister) → 18 ÷ 2 = 9<br>• Tammie → 9 + 6 = 15<br><br>Tammie is 15 years old and her younger sister is 9 years old.<br>Check: (15 + 9) ÷ 2 = 12 and 15 − 9 = 6. ✓