[
  "Draw a bar model: The age gap never changes. In 3 years, Cool Joe is 6 units, his daughter is 1 unit.",
  "In 3 years, Joe is 39 + 3 = 42. 6 units = 42, so 1 unit = 42 / 6 = 7. Daughter will be 7 in 3 years.",
  "Daughter now = 7 - 3 = 4. Answer: 4 years old."
]