The two ages add up to twice the average. Tammie is one unit plus 6; her sister is one unit.

Sister?Tammie?6diff 6total = 2 × 12 = 24

Plain numbers are given; italic underlined numbers are worked out.

  1. Total → 2 × 12 = 24
  2. 2 units + 6 → 24
  3. 2 units → 24 − 6 = 18
  4. 1 unit (sister) → 18 ÷ 2 = 9
  5. Tammie → 9 + 6 = 15

Tammie is 15 years old and her younger sister is 9 years old.

Check: (15 + 9) ÷ 2 = 12 and 15 − 9 = 6. ✓