[
  "Draw a bar model: The age gap never changes. Grandfather is always 51 years older than Sam. Sam = 1 unit, Grandfather = 4 units.",
  "Gap = 4 units - 1 unit = 3 units = 51. So 1 unit = 51 / 3 = 17. Sam will be 17 years old.",
  "Sam is 8 now. 17 - 8 = 9. Answer: 9 years."
]