The remaining 20 km is \( \frac{1}{4} \) of the journey.
Full journey = 20 × 4 = 80 km.
First \( \frac{3}{4} \) = 60 km at 50 km/h:
• Time = 60 ÷ 50 = 1.2 h = 72 min
Last \( \frac{1}{4} \) = 20 km at 80 km/h:
• Time = 20 ÷ 80 = 0.25 h = 15 min
Total time = 72 + 15 = 87 min.
Final Answer: 87 min