Alternatively, we can analyse the travel using laps of 10km for each.
If we read from the question again and understand, Jia Jia who was slower took 40 mins to complete last 10km-lap. Since she maintained same speed throughout, she took 40mins x 5 = 200mins to complete 5 laps of 10km.
Start time: 11:30am – 3h 20min –> 8:10am#
Kai Kai finished last 4 laps of 10km (40×1000= 40,000m) during the time Jia Jia finished her middle 3 laps of 10km (3x40min=120min).
Average speed in last 4 laps for Kai Kai
= total distance / total time
=333 1/3 m/min#