[
  "The remaining 20 km is 1/4 of the whole journey. Find the full distance.",
  "Full journey = 20 x 4 = 80 km. First 3/4 = 60 km at 50 km/h.",
  "Total time = 60/50 + 20/80 = 1.2 + 0.25 = 1.45 h = 87 min."
]