[
  "Draw a bar model: The age gap never changes. 15 years ago, Lance:Father = 1:4 (gap = 3 units). Now, Lance:Father = 2:5 (gap = 3 parts).",
  "Since the age gap is constant, 3 units = 3 parts, so 1 unit = 1 part. Lance grew from 1 unit to 2 units in 15 years, so 1 unit = 15.",
  "Lance now = 2 x 15 = 30 years old. Answer: 30."
]